Daniel,
I made one up many years ago out of 3/16" steel plate and it is still going strong with no deformation using up through BBB shot. It is 4.5' x 5.5' in dimension and sports a four-inch hole cut out of the center to provide a permanent pointing spot at eye level. This eliminates the need to spray-paint a spot for each shot taken. I backed that hole with (i.e. welded) a small 12" x 12" x 4" open-sided, steel plate box that I can also whitewash and count the pellet strikes within the four inch circle as well.
That size and thickness of plate was about all two strong men could hang vertically back then without equipment, as I remember(somewhere approaching 200 lbs.) and has held up well on its 6" x6" treated post framework. If you test 'T' sized shot or larger, you may very well need more thickness to prevent denting. Can't say....
If interested, I can either personally e-mail you pics of the plate when first constructed, or sit down and figure out how to post them here.
Good Luck,
Rob
