When I needed a hammer block when building my 5.56 AR I just went to Harbor Freight and got a cheap cutting board (made of a somewhat forgiving plastic) and cut 3 or 4 L shaped pieces and glued them together with superglue. Then cut with a coping saw/dremel tool, and small belt sander to fit the mag well. I then cut a small mag release slot and a curve to go over the bolt release with the dremel,
Kind of like what this one: (But it could have been made a little longer so that it works as both a hammer block and a vise block)
http://www.brownells.com/gunsmith-tools-supplies/rifle-tools/trigger-tools/ar-15-hammer-drop-block-prod1030.aspx
If I can figure out how to post pics that don't need a url I'll put them here... but for now imagine the brownells' tool out of white cutting board (four pieces thick cut to work as a hammer block. Probably would work as a magazine well block as well.