Interesting concept, never heard of anybody making one RWD but not been around them too long, but there are 2 logical ways of doing it. Nobody will make an adapter plate or I suspect that nobody will, so you will have to find a suitable gearbox & have one made, often the problem doing this is finding a box with a long enough input shaft that can effectively be shortened by 15mm as you lose that length with the plate. The other way if you have access to a milling machine & a Tig welder is to machine the Saxo gearbox housing down & then machine the front bell housing of your gearbox down then weld both parts together so you end up with something like this
This was done to put a BMW gearbox onto a Ford ST engine (I think)
Best of luck either way, keep everybody posted how you get along & let me know if it will be at Stoneleigh to have a look next year.