Fast forward 12 months, add a little bit of "that printer cost loads, you should put some effort into sorting it" type nagging, and I finally decided to make it work. This was at the same time as I realised that XJ frame covers (for the swingarm pivot bolt) are fairly pricey for what they are. Hey presto, an idea was born.
Using a web-based free software called Tinkercad (google it if you fancy a look) I mocked up a couple of designs and set about printing. I think I got to attempt 8 - a variety of alignment, limit switch, temperature settings, layer thicknesses, and plate adhesion, I have finally printed something almost good.
See the orangey looking doohickey in the frame?:

Here's a close up:

You can probably see from the picture that a certain amount of "finishing" is required to smooth the surface, I will probably go for some kind of glass-paper or ultra fine wet-and-dry paper. My previous attempts had either XJ 600 or XJ Rider on them, unfortunately my printer really struggled to print these with any kind of quality so I plumped with XJ.
Here's the actual piece:


I printed in this colour as it was what happened to be loaded into the machine from the last time I tried to print anything. As it turns out its a pretty good match for the wheels, although I have absolutely no idea how it will fare under exposure to sunlight but only time will tell.
The design was built up in layers; the layer behind the one with XJ set into it could be a little thicker so its not quite so see through, but you can't really tell so much once its in the frame.
I also need to change the dimensions for the back of the plug as its a little loose in the frame but thats easy enough sorted.
Once I've got it bang on I'll share the design so if you have access to a 3D printer you can use it.