This is one I made for air gun shooting indoors. It is 19" x 18". Seems to me the 3/16" steel plate was 20" square.
A larger size could easily be made for shooting muzzleloaders with thicker steel, of course.
This one handled a .45 cal air rife just fine as well as 1,100fps with a .25 cal. air rifle.
A simple angled sheet of steel of steel held in a frame would work fine for outdoor shooting.
Mine is box as I needed to collect the fragments. The sheet steel sits on a piece of angle iron, to prevent damaging the floor.
I staple cardboard to the front of the frame then hand targets from the clamps.
My hope is that this gives ideas to you.