Well I scored a sheet of 3/8" plexiglass from work todayand made my first mount with it.
All I can say it without clamps or someone to help you hold it,cutting this plastic with a roto-zip type tool is NIGHTMARE.
The bit's spinning at 20K and with plastic, if the bit is touchingthe plastic before you turn it on, it will chip.
Oh well, I got my first mount made and it's holding prettywell. I would figure 3/8" is plenty enough.
These aren't subwoofers; they're only 90W RMS speakers.