I designed two belt buckles. They use the same mechanism but they look different, so you can choose the one you like most.
One is for 23mm wide strap and the other 28mm. You can scale them a bit at the x-axis if your strap has different dimensions. If you need a significantly different size don't hesitated to ask.

Instructions:
For axle just use filament (1.75). Also place a small elastic piece in the small recess. You can use a piece PE protective foam that you can find in many packages.