C.R. LAURENCE FMS500CH CRL Polished Chrome Frameless Mail Slot. Another CRL innovation is our brass constructed Frameless Mail Slot for commercial "all-glass" entrance doors. This easy-to-install unit features a 1/2" (12 mm) glass flap with a pair of spring hinges that will fit 1/2" (12 mm) thick glass. It"s a beautiful alternative to the traditional aluminum mail slot. Available in four architectural finishes to complement our Door Rail and Patch Hardware. Custom finishes are also available..