Had a play knocking up some of these today.
Few trips back and forth to screwfix for bolts as my calculations the first time were way off compared to none battened seat runners.
Been playing with cheap-ish pine till I come up with a size that suits. (then I'll be asking peter for some more of his glazing offcuts

)
Currently have 20mm rear to 45mm front slope.
I have only done the drivers side so far but what a revelation! I appear to have made more leg room?
The main purpose was to make the seat adjustable (due to the nut bending the rail mine took considerable effort) and to stop myself and the good boss lady from slipping off the seats continually.
I have the passenger side battens made up already and may fit them tomorrow as I will have to take mog out of garage as ours is a tiny single car one.
Quite a simple little addition, but makes quite a difference!
With this in place I can simply rest my left leg on the clutch pedal with no real force and my leg does not rest so much against the gear tunnel! I can now address the wear on the shaft tunnel from the seatbelt buckle as it no longer rubs on the tunnel leather.
I wonder if I could fabricate a tilt adjustable runner base? replacing the wooden battens with a steel frame the seat sits on, I may have to peruse ebay over a bottle of red