First Class Mail Letter Gauge

This is a simple postal/mail gauge to determine if an envelope will fit within a 1/4" thick slot. First class mail is required to be under 1/4" thick per the US Post Office. I got tired of trying to measure the thickness of envelopes, and made this gauge for more durability (vs card stock). If your letter slips through, it can ship first class.
Original tinkercad file is here if you need to adjust for your tolerances:
https://www.tinkercad.com/things/8tOpyhHxY69-first-class-letter-gauge