For modeling RKT, I've always used the same recipe as normal, but without butter.  Typically, it will stand up vertically very well, provided you compress it enough while modeling. Here is a photo of a cake I did last weekend for my son's birthday:

The tentacle is RKT covered in two layers of fondant, and it worked fine.  Here is a Snoopy I made for Christmas, using the same method:

The Snoopy was not quite as smooth, so I used a thicker layer of fondant for the tentacle.  Your Spongebob cake is great, btw!

Those both look incredible. I will apply all of your suggestions and cross my fingers that it will come out good. Thank you for all of you help. I will take a picture and post it on here if you would like to see my end result.

The body on this one is cake, but the rest is RKT.  I melted chocolate, then painted the chocolate on to fill in the holes, then I used modeling chocolate for the skin.

That look awesome.

AI use chocolate ganache over my RKT when i use RKT in my 3D sculptures. It makes the surface very smooth ready for fondant or modelling chocolate. I mostly crumb coat my cakes with ganache, so it makes sense to use up the spare on my RKT :)

I put my Krispies into a gallon ziplock and crush them AFTER measuring them. Get them as fine as you can. I have either mixed them with modeling chocolate for a smooth finish or just coated them in melted candy melts.

Lorillc, Dr_Hfuhruhurr,  Lindseyjhills:

Wow Your sculptures really rock !   You have some real skill (and patience) to make those look so nice.

AThanks MBalaska, that's really kind :)

Thank-you   :)

This was actually my first time using RKT and a stand up cake.  You'll learn a lot here!

Thank you very much for the compliments.  :)

ARunning your rice krispies thru a sheeter helps tremendously. Otherwise squeezing and rolling a lot. The thicker the fondant is will hide dome bumps too. Your cake is great!

Yes, I have heard that many times. For me it is easier to squish in a baggie than dig out the processor, then clean it up and put it away. LOL...it's all about my laziness.

hi, i made a xbox controller out of rice krispies and marshmallows with fondant if you put the fondant on thicker it comes out ok got a pic on my cake central profile gd luck with it.

Here is the end result on my RKT Zombie hands. I learned a few things doing this but all in all, I had fun doing it. Thank you all very much for your help and advice.

One way to make the rice krispies smooth is to put a layer of ganache over the krispies and smooth it out. It costs a bit more, but the effect is wonderful, smooth as can be. I think your SpongeBob is perfect with the krispies.

From everything I have read regarding modeling with rice krispie mix, you should cover in a layer of white chocolate to fill in the holes and cracks  before covering in fondant.

Ugh! Don't smash the rice krispies - don't you want the kids/guests to be able to eat the rice krispie treats?  Not too great if they are smashed up and beat up.  It's really pretty simple people. . . make your rkts by the recipe but delete the butter (just cereal and melted marshmallows). I don't even measure mine anymore - I know when the consistency is right.  Let them set up some.  shape them into the shapes you want.  Melt candy melts (I use a color close to what my fondant will be). Spread the melted candy melts on, let it set up, repeat one or two more times til the bumps are smooth.  Using your spatula edge or a knife edge, scrape the set up candy melts to smooth out any imperfections.  Rubbing with your warm fingers will smooth it perfectly.  Cover with fondant ( a normal layer) and smooth like you would on a cake.  Perfect.  That's it. My Puppy cake has LOTS of RKT parts - the yellow truck has rkt fenders and tires, the police shirt has an rkt walkie talkie, the ring box and lid are rkt.

I smash mine....actually roll them with a rolling pin in a zip lock before I add the marshmallow.   My RKT designs are usually for a topper or something I don't want to make out of fondant.    If they want to eat it...they have to eat it mushed and covered in white chocolate.   It never intended to make sure they can eat it...I do not want the cellulite look so I smash mine.  It still tastes good smashed. 

I don't smash mine at all, ddaigle and there's no cellulite look - they are perfectly smooth, as I'm sure you can see in my pictures. One less step to worry about.  It's simply covering them with the candy melts and smoothing that covers the bumps. Plus they are delicious. . . I have actually had requests for the rkts.  They're that good.  And cellulite look-free. Bonus! 

AIf you use ganache (peanut butter consistency) as Evy911 and I have already suggested, instead of just melted chocolate, you will not need to coat more than once to avoid a cellulite look. In fact ganaching RKT and getting it smooth is a heck of a lot quicker and easier than ganaching a cake. Smashing RKT is one way to get the structure smooth but it's not very cost effective as you will be using more (possibly twice as many?) rice crispies to get the same size structure. In non-smashed RKT structures, the 'air pocket' in each rice crispie is adding to the volume of the structure. Of course if you don't have spare ganache knocking around (I ganache 90% of my cakes so I do) then I appreciate my 'cost effectiveness' argument is moot ;)

ATo make the RKT smooth (as I always forget) run them thru a sheeter down to a 0. Use parchment paper. Run it thru a couple times. Voila! No need for extra fondant and u def dont need an adhesive since the RKT is sticky anyways. No sheeter? Good old rolling pin.

AI realize this is kind of an old post, but I am attempted my first RKT sculpture in a few weeks and was worried about how to make it smooth and how long it would last. Thanks everyone for input. Also, OP, your cake is fabulous!