I used 3/8" particle board for the inside form of my core. Now I'm working on the heat riser. I formed a cylinder 31" high with a diameter of 19" out of sheet metal to give the supposedly ideal 1.5" clearance around the sides of a barrel with 22" ID (due to lack of appropriately sized barrel hanging around here). Then I went to get a smaller piece of sheet metal to form the 8 inch inside pipe, and I realized that the rest of my metal is rolled up in a fairly big roll left over from doing my standing seam roof. Being lazy to unroll and cut, I got to thinking:
What's wrong with a making a 7.25" square inside heat riser form from particle board?
It isn't a hypothetical question-- I made it, but then my wife needed help with the kids and now its late, so I'll see tomorrow when I make the heat riser...