I'm not tracking any M38A1 specific repro panels for the rear deck just yet. I know John Bizal of Midwest Military told me they would have some "soon." You may want to source an old tub out west in the meantime. I've found a few on craigslist for about $500 that have good floors. You may even be able to just swap the body.
If someone knows of a repro company that makes the rear decks, please let us know.
1953 M38A1 MD54216; 20988792; 40-I-160I B-35
1954 M170 MD-A10513; USMC
1964 M416 1/4 Ton Trailer