Mounting 20 30cm by 30cm on a wood sheet

Hi, I would advise using a decent mirror adhesive, something similar to Mirrorbond. We always prefer clear, as colour adhesive can sometimes show through the glass. Try to spread it evenly on you may run into level problems or have streaks show through the glass too. As far as the back board, ideally ply as this will give a better key for the adhesive, but as long as the mirrors are not too heavy individually, MDF would be fine. The thinner the board the more chance the adhesive could warp the board, so perhaps aim for 12mm or 1/2" and above. If you can, bond the mirrors on the board and then let the adhesive go off horizontally. If you can't do this, then you will need to prop the mirrors until the glue has gone off, as its not instant grab. Good luck

I think your issue here is how you are going to fix it to the wall, You will either need to find the studs to screw into or use plasterboard fixings. But even if you screw the mirror onto the wooden sheet you will need to fix that to the wall in the same way. 6mm ply should be sufficient if it is the mirrors I'm thinking of.

id use 18mm mdf

12mm thick screwed