Every mechbox fit ambi seletors. See that recessed circle near the bottom, that is where the transfer gear goes.
Problem is the clearance in the receiver.
You can order the parts you need from wgcshop (only place I found them). They are replacement parts for King Arms Ambi receivers.
You can take your time, a dremell and a steady hand to modify your current receiver quite easily.
You need:
1x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3208
2x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3209
2x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3210
1x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3211
1x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3217
1x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3219
1x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3220
1x
http://www.wgcshop.com/wgc2008/main/...=KA-PT-M4-3218
Don't do the mistake I did and order the correct selector plate (the one in metal). You also need to modify the switch assembly to remove the electrical safety (the two contacts on the exterior) and apply some tape or heatshrink to that section of the selector plate.