Your gonna need to go thicker than .625".
See if there is a sign company or a fabrication plant in your area and ask them if you can't have a base plate that was screwed up on...

Most of these plates are 1.5" to 3" thick, they may just give you one or two... we used to... even if you have to pay 'scrap' price for it... it will cost less than AR plate, plus at that thickness you can shoot it for years to come with just about anything.

Some of these base plates may have flange holes drilled or punched in them... that's ok, gives you someplace to hang them by.

Also tank manhole covers...