A build I'm working on is planned to use a 22" length of BT-70 and the motor mount will be BT-50. Plans are to launch it with D12 and maybe E12. I thought with that large airframe I would extend the motor mount tube to form a stuffer tube and use up some of the extra volume in the tube. I cut the tube to 12" length and made 2 centering rings, the first ring I glued to the front of the tube. So now the plan is to have a 12" long motor mount with cardboard centering ring at each end. I'm wondering if I should have a third ring and locate it near the location of the thrust block. I can do it but it would complicate construction just a bit. I usually like to build things strong so now that I'm thinking about it I probably should go ahead and do it.