Here is how I did it. The ply pad is approx 7″ x 7″ and centred 10″ from the boat centrline on port side. I am right handed.
The pad is made up of 9mm and 6mm ply – the 6mm ply goes to the transom top and the 9mm packing underneath the transom lip to make it flush
The 6mm ply is drilled to take the shoe so that the shoe top sits approx 5mm below the transom top (the top two holes are about 30mm down). The transom is drilled right through for the top bolts which are clear of the tank. Slide the 9mm packing ply under and mark for lower bolts.
Make 2 clearance holes in the 9mm ply to take the lower 2 nuts and large washers. You can now cut the bolts to length and dry assemble it.
epoxy the pads to the boat with the shoe attached. Use the top bolts and a small self tapper at the bottom to clamp it. Sand and scratch the gell thoroughly to get a good epoxy key. fill the small tapper hole
You need a backing plate (3mm stainless fine) for top bolts
Use nylock nuts (locking)
Use high grade ply or tufnol for top 6mm pad (or make it thicker). Robbins Timber “super elite” is suitable – you may get an A4 sample! “elite” is a bit soft
When you epoxy use coloidal silica and don’t clamp too tight to avoid resin starvation
Coat all the ply with epoxy/paint/varnish
Use sealant under shoe and backing plate
Make sure bolt heads flush in shoe so you can slide bracket on!
If you grease the threads you can epoxy the washers/nuts in and should be able to take it apart. degrease the washers/nuts well and they should hold tight if you fill the clearance holes with thickened epoxy/silica when you fasten it all together
While you’re at it two 1/2″ holes through top of transom with inserts are very useful for mooring or attaching a bridle if the mainsheet track pulls off. It happened to me! One hole can be through the shoe.
If you send your address I can send you a sketch if its not clear. This is only one of many methods to fix the bracket and it works fine for me. No need to scrat around inside the tank! My engine (seagull shortshaft) is correct height and the rudder works fine. I put one on a friends with 15hp and he’s had no problems