Id stay away from anything (C)RAP 4,they have great ideas but their productin quality is horrible,try an A5 with mag conversion if you want something resonably priced and reliable,there are a few for sale at M Carter Brown on occasion,dont give up on your ideas just start with a good base marker,heres a thread for my most recent Ion boxmag
http://www.pbnation.com/showthread.php?t=3868733
there are a few MCB ers that have made similar ones for different types of markers.
If you decide to try again and have any questions feel free to PM me,good luck