Hi Paul ......
No good news from me I'm afraid, but I'll tell you what I know. You may already be aware but there are many others out there also in your position.
Finding a back bonnet in UK seems to be a matter of being in the right place at the right time, unless there is another reader out there who is sitting on a nest of them all and isn't getting around to digging them out.
The problem with buying one sight unseen whether from a fellow collector in UK or a dealer in Americaland is they make look right. However until you offer it up and prove it fits you may well be in for disappointment. They do vary, and if it has come from a clone or is genuine but your machine is a clone then it's a fair likely hood it won't.
Two of us have been researching into having a batch cast.
Unlike Back doors and Cash Boxes, this is proving to be a proper Faff!
Doors and boxes are bent mild steel which is cheap to get and easy to manufacture. Bonnets are Aluminium/Alloy and cast. This is expensive and a specialist job.
The Bonnet itself is made of at least 4 pieces. The Shell, a mild steel reinforcement strap rivetted in place and two steel pins which the securing brackets pivot onto.
Currently a single bonnet appearing for sale usually goes for between £60 - £100+. We're being quoted a fair whack more than that, before any mark up that would make it worth doing.
Then there is the issue "Which model do you replicate?" and how many of them will bounce back because it didn't fit?
I'm not giving up, but I'm not sure about the way forward. GRP would be cheaper and easier but I wouldn't be impressed as a collector.
There's clearly a market but from what I can see the solution is a bit of a kerfuffle.