I'm doing just that with 54mm spent reload case liners, I'll post a pic once I take one of it, the fins are from the darkstar lite kit and for the nose cone I'll be using the spent medusa nozzle. I'm thinking a 24mm MMT will do it, this way I can use Estes motors as well.
Can you even do that? What is the inside surface of the tube like? How do you remove the aft and forward closures? Where would you find parts like nose cones? The BT would be the ID of a standard BT. In other word, a spent 54 mm tube OD would be the same diameter as the shoulder of a 54mm nosecone.