Diff gasket? NLA?

[Black88GTA]

So the '92 I'm fixing up had a Level 10 4.44 transmission swapped in back in 2005 (which I actually do have the paperwork for), and it seems like it is working well. According to the logged mileage, this trans has less than 10k miles on it (!) However, the paperwork states something along the lines of "informed customer that rear diff swap to 4.44 gears is required" indicating that this wasn't done, at least not at the time the transmission was. I don't have any paperwork showing a rear diff swap...and I've got a bad torque binding problem I need to figure out.

Before messing with Duty Sol C or the driveshaft itself, I wanted to open up the diff and see if I can ascertain exactly what's in there, what condition it's in, and swap it with the 4.44 in the parts car if needed. But, looking around online, it seems that diff cover gaskets for this car no longer exist? I found the part # for the gasket from Subaru (383385200) but it comes up as NLA (of course), and parts stores / Rock Auto don't seem to have any either.

What's the best course of action here? Just plan on putting the cover back on with RTV only? Try to roll my own with a sheet of gasket material and a box cutter? Never been much of an arts n crafts guy, but I could give it a shot I guess. Already had to do that with taillight gaskets