Using RKT to form a shopping bag. It will be approximately 6" x 8" x 12" high. I do not have a form that size. Can I form it by hand (will it keep its shape) and then carve the rectangle? Or do I need to create multiple layers and stack them. I'm going to add chocolate to the mix to make it firmer.

In a pinch I have used a cardboard box lined with wax paper. Once it set, it slid right out and i just peeled the wax paper off. HTH!!

That's a great idea. Would it bulge the sides of the cardboard out? I've not used RKT before for cakes. How long might it take to set, given a recipe with little butter and chocolate added?

I used a shoe box and didn't have any issues. See if you can find a heavier cardboard box that is just about the size you need. You can always cut it down some if need be once it is set.
I let mine set overnight....just to be on the safe side.

where do the big cake shops get their rkt? on tv they are always the same size blocks do they make them or buy them somewhere?