Open Easel file to see all the pieces needed on the top of the ramps.
My boards ended up with 70 × 53 cm and the overall size of the machine is 140 × 55 × 20 (in the middle where the ramps are higher) cm .
I cut the middle angle on the boards’ edges to 8º. It seemed to be a good inclination for the balls to roll but you might use a slightly different angle. The steel balls I used are 22mm in diameter.