Actually FWIW most acrylic is going to come in metric thickness. The acrylic calculator I have reccomends .354" with bracing, and .472" without. If you do end up getting material that's not metric thickness, 1/4" would actually be a bit too thin WITH a brace (.237" min), so you would need to go up to 3/8" with or without bracing (.356" min without bracing).