I designed this as the available ones are either too flimsy or too tight to be functional. Also very few specify dimensions.
I also added fillets and chamfers for strength and aesthetics.

For fitting both these have 5mm countersunk holes 30mm between centres.

The first set works fine.
The male part is 10mm diameter.

The second optional male part I have included is 10.5mm to make it a slightly tighter and secure fit.

Feel free to print both and choose which you prefer.