I contacted boats n bits and they will sell just the bracket bit that fits on the boat for “around £20”
So you can get the other bit made in a welding shop and vary the angle of dangle to suit your outboard length (and probably save money)
The tapered plate on mine is 3.2mm thick (1/8″). 3mm would probably be fine. The length is 6 1/16″. The width top is 4 5/8″. The width bottom is 3 3/8″. There is a tapped hole to take a 6mm bolt with wing nut approx 1″ up from the bottom at the centre. Check this on your actual bracket. Round the corners and edges.
The length of the arm is 8″. of 2 1/4″ x 2″ channel 1/8″ thick but box section, pipe or whatever would be fine
I hope this is clear and will suit everyone. Apologies for delay my computer has been down. Any queries please contact me
The plate at the other end should be suitable for mounting your plywood mounting (approx 1 1/2″ thick)