Danny's recommendation is 3/8" which is right in the middle of both 1/4" and 1/2"
But 1/2" will be fine. so long as you're using 3/4" MDF for your panels.
you can mitigate any weakness from 1/2" by adding small wedges to the corners, or adding additional glue along the inside edge (so long as the wedges don't interfere with the woofer cut out, which also needs 3/8" round-over on the inside edge)