I designed a wrist rest that is made out of 4 pieces to fit my 65% keyboard. The dimensions are 32cm x 8cm. The height of the top is 2cm and at the bottom it is around 1.2cm.

You can print the middle piece as often as you want to make it longer/shorter.

The pieces are connected with 3d printed dowels but I also upload the version without holes (if you want to glue them together for example).

Another version is the full wrist rest in one piece.

I sanded mine to make it more smooth, it's not perfectly sanded but good enough for me.