i made one. got a sheet of 16 ga. galvanized from the big box store and contact cemented it to a sheet of birch cabinet plywood. grabbed some cabinet door magnets, and can hold a bunch of stuff square, paralell, & perpendicular. whenever i have extra pieces of angle or bar, even framing clips for house construction, i stick them in a cigar box for a possible jig up.
it's the same thing that is sold for the purpose, just made in the shop.
some good stuff here:
http://www.airfieldmodels.com/information_source/how_to_articles_for_model_builders/tools/magnetic_building_board/steel_building_boards.htm
http://www.airfieldmodels.com/information_source/model_building_tools/jigs_and_fixtures.htm