I've made several using stainless shim stock and nuts & bolts.

mcmaster.com part #9506K14 or 9506K16

Figure out the circumference and add a 1/2" lip. Drill 3/32" holes every 1 inch and bolt together with 1/8" 2-56 stainless flathead bolts with washer and nut (bolt head inside the pipe for less drag). Make the outer pipe about 1/4" larger in diameter all the way around. Use 1/2" 2-56 bolts & nuts at 4 points for standoffs at each end. Be sure to offset the inner pipe around an inch shorter than the outer at the exhaust end. The outer can be a few inches shorter than the inner at the inlet end.

Make sure you use a bellmouth at the inlet or a bolt on few wraps of stock at the inlet so the pipe will not collapse. If you don't, it will.

Note, most pipes taper down in diameter at the exhaust. You can do that or make it straight. Done it both ways.

If you use the same thickness shim stock for inner & outer, you can make a pipe up to around 28"-30" with one roll for engines up to about a P-80 at a cost of maybe less than $40 and 1/2 to 1 days work.

I've done several hundred flights on these homemade pipes and they will hold up.