Dogdustr -
WELCOME to the board!
You're already off to a good start: 1. you have the material for the tube and 2. you've established the caliber.
NEXT: start dreaming:
If I had my 'druthers I'd like to have ____________.
Mortar? Union or Confederate.
It's long enough for a howitzer style.
Establish your priorities of what's the apple of your eye!
Some things are 'given' - powder chamber, thickness of material around the powder chamber, et al.
All the rest is what you WANT.