Holy ****! Those are some incredible reference pics!!! This is something i've always thought of doing since the first episode!
I thought at first that the front piece that the Thompson handgrip is attached to was a single rail system like the ones that are sometimes used to replace the front handgrip/barrel shroud on the ArR15/M16 series rifles, but it could easily be a custom piece with seperate rail pieces bolted to it. Again, I could certainly be wrong.
I've checked some prices, for a Thompson airsoft, nice one and the rail system they both come out to about $500 then you have the custom built isotope container, barrel, and middle section, kinda pricy, but I think if I ever get the cash I'll try to assemble one