How do I measure for an inside mount?

Use a steel tape, not a cloth one, and measure the opening itself — don’t make any deductions yourself unless the seller’s guide explicitly tells you to. For width, measure at the top, middle, and bottom of the opening and record the narrowest of the three. For height, measure at the left, center, and right and record the longest (or per the seller’s rule). Also measure the mount depth — the flat distance from the front of the opening back to the glass or any hardware — to confirm the frame is deep enough. Report those exact numbers to the seller and let their factory apply the deduction. Because custom sizes are generally not returnable, confirm every step against your seller’s measuring guide before you submit.