So I've been scouring Ikea and other furniture suppliers to try and find a replacement for this. Yeah I could just get some MDF board, but then I'd need to cut it to size, paint it, etc. I came upon
this drawer that looked to be just right, but annoyingly the stated internal dimensions are actually the internal ones.
A quick rejig by mounting the sides differently (see attached CAD) and it fits some Tip Top 84hp rails quite neatly, probably up to 7U.
The tops of the side panels are exposed particle-board, but some stickers sorted that out. You could still use the bottom of the drawer as a back panel, but it won't fit the full width. Another option could be to get another one and attach it to the bottom with the supplied dowels and some glue for 14u for £30.
Since I've only got 1 row of modules at the moment, I don't have much need for more than this, but someone should take a look at
this kids storage unit, I reckon it might fit some 19" rails.