Find your self a suitable gauge to mount into a standard pod, This Revotec one was £8 on Ebay and the clock was £12.

Disassemble the clock / rev counter and remove all the guts as you won't be needing them.

Using a Drimel or simualar cut a circle the same size as the gauge in the front of the clock glass (see above)

Cut a hole on the back of the clock / rev housing to feed the vacuum pipe a connector through, Push the gauge through the hold you have just cut.

Turn it over and fix it in place with a large rubber band (a small Livestrong band will do nicely) then push the connector through the back of the gauge.

Clip the housing back together with the gauge inside locking it all in place.

Routing the boost pipe can take some serious imagination on a manual gauge, Digital ones would be simpler as a control box could be wired to a more suitable location where more room is available.

The pod can be added as an extra item or as a replacement for one of the others.