I got this one for the back:
Amazon.com: Galaxy S8 Plus Screen Protector (Front and Back), amFilm Full Screen Coverage Wet Application TPU HD Clear [Case Friendly] Screen Protector for Samsung Galaxy S8 Plus: Electronics
Didn't use the front one since I have the Neoflex for that.
Works fairly well, one minor air bubble but a few hours after installation, it's crystal clear.
Do note that it says wet install and is supposed to include instructions, but it didn't include instructions or the wet install spray thingy, so I used the one for the Neoflex on the front and back and the protectors are fine. No peeling off or anything.
My grandfather has a Spigen case on his iPhone with the magnetic car holder and it works fine.
Sent from my SM-G955F