The plexiglas covers used in amp were designed to have the same size of a Hammond 1590B which is a common enclosure used in guitar pedals.
So if you decide to put your 1Wamp in a metal enclosure or you have any other effect in this box size you can use the transparent covers to illuminate them.
1 - You would need to file the ledge on the lid of the 1590 enclosure. I use a standard metal file and takes me around 20 minutes.
2- Drill 4 extra holes on the plexiglas cover: Its pretty easy, just present the plastic over the enclosure and mark the position for the 4 holes. A 3mm drill bit would do.
3- Just screw it and done!
PS: The LED should be mounted upside down to light the bottom side of the PCB. IF you want some extra illumination you can place more LEDs in series (no more than 2) or in parallel.