I've not got a design but have built one for my 4" Ruston Proctor SD,
This is Mk1 with a puncture proof tyre, this wheel was not man enough for my 12 stone. Steering on hard surface was good but on loose surface very poor.
Mk2 Twin pneumatic wheels and footrest position changed, engine raised on blocks with scales under the front to check the downforce changes. The seat can also be moved for and aft to position bodyweight. Working better but still a little light at the front on a loose surface.