This is s 40" diameter rocket with an approximately 6" diameter motor casing.
1.) There's no need for any motor tube.
2.) There's no need for a super tight fit for the motor casing in the centering ring.
3.) The motor retention on the aft bulkhead will secure the motor to the bulkhead, not a motor tube.
4.) The upper bulkhead/centering ring need only have a hole with sufficient clearance to hold the motor upon insertion until it is secured by the motor retention
system.
5.) While the motor will have about 2000 pound of thrust force, it is distributed over a very large cross-section. The rocket could be built entirely from fiberboard and not even come close to structural failure loading. For example, fiberboard honeycomb panels such as
https://www.packitusa.com/Search.aspx?s=honeycomb are used to support and separate stacks of filled 55 gallon drums. For example a 40"diameter x 2" thick fiberboard honeycomb bulkhead with a 6" diameter centering hole would weigh 3.2 pounds.
https://www.packitusa.com/Shop/PHC40482-C229.aspx It could be stiffened by (6) 17"x6"x4" fiberboard stringers weighing about 3 pounds attached to the interior side of the bulkhead obtained from fiberboard honeycomb pallet runners.
https://www.packitusa.com/Shop/PHC4864R-C240.aspx
A single pallet runner could be used cut and used to fabricate 6" wide by 6" deep by 32" tall stringers to support and space the upper full diameter rings. 6 stringers would be conservatively rated to carry 2100 pounds load at 10 psi which a weight of 7.5 pounds.
The mechanical framework core of a 30' long x 40" diameter airframe would weight about 11x3.25+1x3+10x7.5=~114 pounds using this method.
