On my deck I loosely covered the 4x4 deck posts in cedar, so they're about 5-1/4 x 5-1/4 inches, varying by 1/8 or so. Deck post caps cost upwards of $5 each, and I couldn't even find ones the right size. 3d printer to the rescue, custom-making them a few at a time to fit the different post sizes. Because of a reddit thread I thought I would put a few of them here.

If these are useful but you need different dimensions and use OnShape, feel free to copy my document and adjust the postSize variable.

https://cad.onshape.com/documents/84a81322bdef425c694bda12/w/ffeb1c96fb6544efcf52c51a/e/fb121cad4b4a448954a2d187