If its a one-off, or <5 units, you'd be surprised how good hot-melt glue can be for diffusing. It's also easy to squirt into molds, and cut with a knife. It'll hold itself in place, but won't be adhered so much you can't peel it off. I realize 'hot-snot' isn't the nicest solution, but I've used it before in small areas (<5mm) and it diffused it well, and allowed plenty of light through. I let it go through the hole, then sliced it off so it was flush with the front of the case with a very sharp blade. At the back I used 4mm M8 spacers as a 'tube' so the area was only lit form an LED directly below, and not the others.
A more permanent would be clear epoxy. If the LED is directly below and you don't need 90 degree angles, or wacky shapes, you can parcel tape over the hole (the brown stuff, shiney side, epoxy won't stick to it) and use something like a plastic standoff as a 'tube'. It'll be completely clear, but when roughed up with 400grit, it diffuses well.
Of course, the above is very much a cheap DIY job, hardly professional. In fact out of the two the hot-melt looks the best and is the quickest to do.