This is a remix of the popular hold down clamp created by WorksBySolo. I made a few improvements to the original design.
The long edge of that design was too long for my purpose and it snapped on me a couple of times, so I:

• Shortened the length of the clamp
• Made the bottom curve smoother to eliminate weak points where it could break
• Added fillets so some amount of elephant's foot can be tolerated due to improperly calibrated 3D printers. One wouldn't need to sand after printing to ensure flat and even clamping pressure

I printed with PLA at 30% infill without supports with good results, but PETG would be a better choice as it's more flexible.