What can be done, and it can be one with nothing but a hacksaw and a 1# tank mapp gass torch, is to find a piece of iron pipe that is about the same ID as your trunnions. Cut the pipe in half, braze two ears to it with the torch and brazing rod, and you will have some steel trunnion caps.
Look back a bit at my Tropico's and my mini-dictator mortar and you will see similiar construction. I think Tropico maybe welded his, I brazed mine.
For my 1 " brooks naval cannon I welded instead, but brazing would have worked as well.
I agree with GGaskill, the caps just hold the barrel on and don't really do much as far as recoil goes. That is especially true for a mortar. 20 G copper is very thin, but it would do the job if that is the way you wish to go.
I guess you could even make caps out of wood if you wish, possibly capped with a copper 20 g plate to help prevent it from cracking.
To hold the caps in place on a gun of that size, I have been drilling into the wood and JB-Glueing some threaded stainless 1/4-20 rod down there. The rod goes thru the caps and I use brass plated acorn nuts to hold the caps in place. The rod goes deep into the side plates, doing double-duty as a reinforcement for the sides.
