Dunstall 1/2 fairing

Have you tried Mighty Putty?

Are they fixed in place with a u-shaped (with 90 degree corners, if you see what I mean) brackets on the back, like dials on classic cars?

I'm sorry I don't have pictures, but have a look at Ebay item 390345266482. It's shaped more than you really need - it just needs a flat middle section and straight sides, so is easy to make.

The back needs to be slightly wider than the instrument case, and the side 'arms'approx. the depth of the case up to the bottom of the bezel.

The middle section bolts on to the back of theinstrument (usually a couple of mounting studs or screws on the back of the case,) after theinstrument is placed in the hole of the dash panel in the fairing. The side 'arms' then bear against the back of the dash panel to secure the instrument in place.

Also see Ebay item 110854559069 for a variation on the same theme.

And also Ebay item 400254487978. (I'll stop now - I'm sure you get the gist!)