Clip for straightening curling Costco calendars

My family makes and receives photo calendars each year for Christmas. We've been using the 14x11" calendars from Costco but the corners keep curling. This is a simple U-shaped piece with a hole for a nail/screw meant to slip over approximately half of the calendar at the top and another piece to slip over the other half at the bottom. Helps with already curled pages; will hopefully prevent any curling when used on new calendars.

There are two versions. The original is thinner (0.8mm walls) and a little fragile freeing from the supports but works fine and is faster/cheaper to print. The “thicker” version has 1.2mm walls and is less temperamental but takes a little longer/uses slightly more filament. The last photo has both versions side by side.