Some of the early OTS builders decided to mover the OD trimmer to the rear panel, myself included, and this allows you to explore the possibilities much more easily. You are right that the hole is too small for a standard pot and since I did this mod myself I just went out and bought a small pot. I guess Nik has supplied a standard pot and you will have to drill this out if you want to do this. Now this will prevent you installing a Full/Half Power switch in this location so you will have to make a choice here, or find an alternative location for a Full/Half power switch. This switch hole is I believe a hang over from the original D*mble which was 100watt (see below) You could easily move the Mains and Standby switches, utilising the now now longer used Ground switch hole, this would free up a switch hole next to the OD trimmer.

Before you make that decision, the 100w versions have a simple half power option which takes two of the 6L6's out of circuit. The will possibly be a slight reduction in tone (I have yet to A/B both versions side by side but when I have used amps with this facility it always loses a very slight something on half power, it sounds less 'big' somehow), but it is an easy mod. If you are building the 50watt version the only power switching option you will have is a Pentode/triode switch for the 6L6's. If you are an obsessive about tone don't waste your time with this, it is a big tone sucker! Consider getting a good attenuator instead. If you do want to do it you'll have to contact Nik for the layout drawing.

Have fun!