I've gotten a bunch of requests for this, so thought I would post instructions. First of all, all credit and many thanks goes to kiraboo for the original idea.
Start by stacking sheet cakes into an 11X15 rectangle (I only had 9X13 pans, and so baked three and cut them up until I got the proper size.) Make sure to bake them high enough, as you'll need some height for the ice cream cones to fit on the side.
On one end, carve out a semicircle shape with three arches for the crayons. Again, be careful not to carve away too much of the cake because you'll need enough height to comfortably stack two rows of ice cream cones.
Cover with buttercream and fondant.
Cover ice cream cones with fondant. I used aine2's edible glue (water and tylose) directly on the cones and then covered with fondant. I found that rolling the fondant thicker prevented it from ripping. It also made for a smoother finish, because it covered up all the designs and textures in the cone.
Smooth with fondant smoothers--this is the only way I was able to get it smooth.
I used melted chocolate to glue the cones on the side. You don't need to fondant the end the cones are stuck onto, because the cones will fit closely enough that you can't really see the small areas between them. If your cones don't fit, you can shave off some of the sides to make them fit. I had to do this, but you can't tell once the cake is finished.
Finish with your fondant decorations, including gumpaste flaps for the lid and side. Photos of the process follow (please excuse the mess in the background!).
Good luck with your crayon cake!