The Brey Krause products are hands down the best option for the E8X/E9X and your search should start and end here:
https://www.bkauto.com/category_s/319.htm
I had one of each style, a "Direct to Floor" seat mount (aka no provision for a slider) on one side and the "OEM Slider" mount on the other. They both worked perfectly with the stock belt buckle/tensioner assembly and it's really nice to be able to reuse the factory slider from both a cost and safety perspective.
It is far cleaner to have each mount be a single piece rather than the Macht Schnell style with a floor plate, then a slider, then a side mount, etc. I genuinely cannot recommend the Brey Krause more - I thought they were fairly priced for being a perfect bolt in solution and just worked as expected.
A bit more info on the weights on my thread here:
https://www.1addicts.com/forums/show...&postcount=117
-Mark