No disrespect Wavy, but in this case your wrong. I have built seven composite boats and all of them had components of marine ply (not cheap exterior stuff, it must be structural). First off, good marine ply is one of the most dimensionally stable things you can use. Which is why it is used in high stress load applications in boat building. You want to use at least 18mm when building a mirror base. It does need to be sealed with a good boat building epoxy but it is a good mirror base. Epoxy encapsulated Marine Ply is a composite. Plenty of scopes have been built with the Stellafane mirror cell plans.
https://stellafane.org/tm/dob/ota/cell2.html
I used a variation of this design to build the cell for my 10" imaging newt, which supports my full thickness mirror perfectly.
Side 90 degree L brackets are fine, leave a gap and inject silicon through the bracket hole so it just touches the mirror side, once cured no movement possible and it won't fall out. Pretty sure the photos from my Newt build are still in the ATM forum here.