Check out this part from Marlin P Jones

http://www.mpja.com/10K-Ohm-10-Turn-Variable-Resistor-with-Dial/productinfo/32676+VR/

http://www.mpja.com/download/32676VRMech.pdf

It's a ten-turn 10K pot with a "clock face" dial. $6.95
To use it as a replacement for R8, just change R5 to either 6.2K or 6.8K
The fact that it's a wirewound pot is not an issue, as there is only DC on R8.

The problem is that it projects a bit over 1" above its mounting surface, so some pretty good surgery would be needed to the panel to allow the box lid to close and latch. It would have to be recessed into the panel by 3/8" or so.

N5IB