I have made them, made my patterns
I cut two disks 7" in diameter in 7- 8 oz and two 6" diameter also in 7-8oz
I glue and sew the disks one inside of the other
I cut my body as rectangle the length ( of the "Portmanteau" )say 24" X the (6" Dia X pi {3.141} 18.8496" add 4 '' for your flap.
Or in some cases I'll scallop the leading 4 1/2" of 8-9 oz and sew this to form the flap closier.
I sew 3 straps & buckles.
You can tool the end caps ( on mine I was coping an original which had two sew 1/2 moons (@ 3 and 9 oclock) and two full moons's (circles)@ 6 & 12 oclock.
The original was a saddle vallice, I used it that way for years, last year I added a grip handle, and I use it an ammo pack at NCOWS shoots.