If you're going to have a boost gauge it needs to be somewhere that is close to your line of sight, it's no good having it down in the centre console because you'll never look at it.
If your booting it in 3rd then the last place you want to be looking is down
I know it's in a Mk4 and not a MK3 but mine is mounted in a cheap halfords pod on the dash. It fits perfectly into the demister vent, iirc the MK3 has a dash speaker around there? Would be possible to modify that to take a gauge holder.