Auxiliary Center Diff??

Does anyone know where I could buy an external center diff?

Or, have any ideas for "making" one?

I'm building my '80 Hatch into a street racer of sorts (autocross?), and I'd like to eventually put a FT4wd or AWD 5spd into it for better power distribution. Since the FT4wd trannies are a little hard to get, I wondered about the possibility of creating FT4wd by adding a center diff to a PT4wd tranny?

I'll probably stumble onto a FT4wd box and use that, but I'd like to explore this possibility.

So far, my only idea has been to modify and mount a Justy rear diff sideways... Not too practical, but it could work. (I could weld one of the side gears to the carrier and remove the pinion gear completely, then cut the front housing down and cap it off.)

Just dreaming here, all thoughts are welcome.