Ok, I think I understand.
You want to make parts FOR an airsoft gun, not make an airsoft gun.
Making parts is ok, making an entire airsoft gun is not. So making a new kind of mechbox is fine. If you make a new frame... that's pushing your luck.
The only license you could get would be a real gunsmith permit plus the extra paperwork to allow you to work with Prohibited devices. But that's only if you want to make an entire gun.