[XMetal]

My washers are doing the same thing (dished) and I'm using 2 of them on each stud! I think the problem is a combination of the oversized slotted holes on the tower rings and the acorn nut not having large enough contact surface area to the washer. Therefore placing all the clamping force in the middle of the oversized slotted holes instead of evenly on the entire washer, essentially pushing (squeezing) the washer down on the oversized slotted holes.

I decided to experiment this weekend and just went out and bought some automotive M8x1.25mm flange nuts with a 17mm outer diameter that fits in the countersunk holes almost perfectly (I think 18mm outer diameter will fit and is a bit better if you can find that - I believed the OEM flange nuts are 19mm outer diameter which will NOT fit in the countersunk holes). I think the best solution would be if the countersunk holes can be bored out a bit to fit the OEM flange nut.

Note in the attached pictures how much more surface clamping area you get with the flange nut compared to the acorn nut.