Stainless Steel. I've used two products that work well. I'm sure you'll get other suggestions as well.
2018 North Point 377RLBH, Roadmaster Comfort Ride, Dexter E-Z Flex Suspension, Flexair 21k Pinbox
2017 Ford F350 Lariat CC DRW LB 6.7, Air Lift 7500XL with Air Lift dual channel wireless