Just adding some photos to this thread to update anyone interested in the progress.
The project is advancing in the right direction. But I'm heading down a path that I think a lot of people end up on (based on my research here on the forum). That is, basically sculpting the foam to suit my butt
Stage one was to trim the edge off the stock seat.
Stage two was to add temporary foam and experiment with the shape.
Stage three will be to get new foam professionally shaped and fitted, based on my experimenting, and then having the seat recovered.
For what it's worth, I've learned three things:
1. This is a really individual thing. I don't think there will be a 'mod' (like the 'seat-lift mod') that will come out of this to suit everyone.
2. The stock seat pan has some significant limitations, therefore the final result will have some compromise involved.
3. This is a game of millimetres. Small changes make a BIG difference (that's a positive thing given the limitations involved)
Having said all that, I have managed to relieve the pressure on the tailbone, and created some wriggle room on the seat. I will be making some minor adjustments and then going to see a professional to finish it off.
Here are some photos of what I've been up to.
I cut a couple of pieces of camping/sleeping mats to shape, and stuck them together to create a thicker layer under the drivers seat: