Boxing in a 6" dia outlet

Personally I would use plywood (or MDF, if I had that to hand) but probably 9 or 12 mm: easier to work and fit, more rigid so should not vibrate and will look nicer. With 6 mm you need a batten on the inside and probably a moulding on the outside to make it look tidy.

You *do* already have a sawboard, I trust? If not, see wiki.

In article , Nthkentman writes

The ply will be easier as the long edges wont need extra finishing (as long as you put a small lap on the horizontal to cover the sides).

Personally I would use plywood (or MDF, if I had that to hand) but probably 9 or 12 mm: easier to work and fit, more rigid so should not vibrate and will look nicer. With 6 mm you need a batten on the inside and probably a moulding on the outside to make it look tidy.

You *do* already have a sawboard, I trust? If not, see wiki.

Nope.... But the local "Shed" I am V V friendly with has a lovely wall mounted saw unit..... Amazing what a bottle of wine will help with

Well, that's OK if you have flat walls. Mine are wavy, so often the first job is to scribe and cut an oversized board to fit the wall, then you markup the other (outside) edge at each end where you want it to be, then cut with the sawboard. That lets you construct square structures which fit into irregular spaces.

But really, for speed of accurate construction of stuff out of sheet I cannot recommend a sawboard too highly.