Heating the AR plate, be it flame cutting it to size, or welding it to something else, will soften the plate on the edges where the heat is applied. Here is an article on welding it:
http://www.thefabricator.com/article/consumables/cracking-the-caseIf you have access to a stick welder and you want to save some money, you could could lay down some beads of hard facing material on some ordinary steel and use it as though it were hard plate. Not practical for really large areas, but for a few square inches maybe an option. The hard facing is abrasion resistant.
http://www.lincolnelectric.com/en-us/consumables/hardfacing/Pages/wearshield-stick-electrodes.aspx Finally, if you have access to a forge of some sort (know anyone who makes knives or shoes horses?) you could get a piece of high carbon tool steel and do the heat-to-cherry-red, quench, heat slowly to blue then set aside method to create a piece of hardened steel. Or, even with mild steel you could case-harden it
http://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/color-case-hardening/surface-hardening-compound-prod27119.aspx which would be better than nothing.
You could get creative if the hard steel plate is not working out. For example, make a slurry of aluminum oxide (blasting grit) and epoxy and put it on a sheet of copper and repeat a few times to make a sandwich then back it up with some sheet metal. Could make one layer of small ball bearings in epoxy. Copper would grab drill bits and the aluminum oxide would dull drill bits. Or buy some cheap files from the hardware store, cut them (break them) to size, and use them for hardplate.
