You could try loading a spare mag with empty steel cases then wrap it in duct tape and use it as a mag vice block, that's what I did for my first AR with an old GI aluminum mag. Not as solid as the delrin AR blocks but better than nothing. Eventually I picked up a Tipton universal gun vice at a local sporting goods chain for $40 and cut out the stock support hump to make room for a pistol grip, works with my ARs, AK, VZ, Storm, and everything else.