A Sullivan 32oz tank fits easily behind the main fuselage bulkhead (behind the main tanks). At first this looks like it might cause an aft cg issue but in fact this location is very close to the cg. I have been using this setup with the TAM smoke system and it works great. Good for about 3 or 4 minutes of thick smoke. Some guys add a small header tank which you can put up front or even in the wing root but I haven't found that necessary.

I have heard that Gary Mueller (sp?) makes a 50oz custom tank that also fits in this area. That would probably be ideal.

Craig