Hi lulbell..
No... they don't need angles washers at the rear..
The entire seat rests on a carpet..
No solid support under the seat.
If you've ever done seat slide bushes, you will know that the seat rails are seriously thick metal and extremely strong.
If they weren't, the body weight of some people would now them between front and back mounts..
I did look into your concept when I designed these seat lifts in conjunction with an auto design engineer at Land Rover at the outset.
He was largely responsible for making them a very safe and supportive item which provides the correct amount of lift so that if you have a tall person in the driver's seat, and the seat fully reversed on the seat slides, with the backrest fully reclined, it will just allow a 20 mm gap between backrest and the rear body panel behind the seats.
I had the thought about the angles washer for the rear, but he calculated that across the length of the rail, it would need to be just 2,4mm thick at it's thickest edge.
You'll agree it's somewhat OTT to consider that option.
Anything thicker at the narrowest edge, will actually lift the seat higher at the back, meaning the person's head would be even closer to the roof than it already is under normal circumstances.
So, that silly thought was expertly discarded.. thanks to my helper at Land Rover..