The plywood I used was 18.7mm. There are 4 inner pieces and 2 outer pieces. The total width of the inner pieces was 74.8mm. Depending on the with of your ply and your requirement of blade width, you can decide how many inner pieces you want.
Note the height of the castor wheel. I’ve given a gap of 3mm from the ground level. If your castor wheel is of different height, then you will have adjust the height of your pieces too.