As much as I love 3D printing, I still don't fully trust it to support my 30 lbs. backpack. So for all of those out there like me who whose backpacks undergo more trauma than a HALO parachutist, here's an indestructible buckle.
New and improved adjustable backpack buckle. Works with flat straps up to 26 mm wide.
It uses 5 mm thick Delrin, but other materials can be used.

1. Lasercut each of the components from a 5mm thick material. You need to cut two side plates for a total of 5 pieces.
2. Assemble using a >35mm screw. You can either tap the material or use a nut on the other side.