I wanted to better be able to tell line of site when playing 15mm scale war games. I bought a bag of 25mm round mirrors and threw this design together. You may need to modify it for your uses depending on the size of your print bed and the size of your mirrors.
You'll need to glue the mirrors in some how to keep them in the slots after you slide them in. I plan on using a small bit of filament glued in the slot as the slot is a touch over 2mm and filament is 1.75 that should work out ok.
Do not use super glue as that tends to leave a white fog on the mirrors.