This might help narrow the search down a bit...
The TH700 and TH400 covers were identical (from what I was looking at) so you may be able to source one of those for the Diesel.
Second, My 4L80E cover off my '96 tahoe and the '95 suburban (back in the day) looked like they would interchange with the older Diesel tranny.
Maybe this helps.
I'd like to compare the parts.
I think I have both in the shop to look at, but they belong to a tranny I'm going to use, so no spares, or I'd help you out for sure.
Don't source one for a Hummer though. A recent job working on an H1 revealed the use of just the thin stamped-steel cover. I wouldn't own a POS Hummer now though!
J
1982 C10 SCSWB 6.5TD, mods too extensive to list. (13.69 1/4 mile @94.6 MPH) RACE TRUCK
1982 C10 SCSWB 6.2NA, 2.73 700
1986 C10 SCLWB 6.2TD 3.73 700
1989 V20 SUB 6.2NA, 3.73 400
1994 G20 VAN 6.5NA, 3.42 60E
1994 K20LD ECSWB 6.5TD, 3.42 80E
1995 K20 SUB 6.5TD, Wrecked, ran into by stupid teen.
1995 C3500HD DRW 6.5TD, 12' Flatbed 5.13 80E
1995 C3500HD DRW 6.5TD, 18' Rollback Wrecker 4.63 80E
1994 C20HD ECLWB 6.5TD 3.73 80E Wifes Truck.
1995 C20LD ECSWB 6.5TD 3.73 80E
1995 K20LD SCLWB 6.5TD 3.73 80E
1996 K30 DRW 6.5TD 4.10 80E
1997 C10 Tahoe 2Door 2WD 5.7L to 6.5 Conversion Underway