I was looking for a set of ramps to make it easier to get under my 30 ds for doing oil changes and installing my suspension mods. The strongest set I found were a plastic set rated only for 16k gvw.
I ended up building a set from 2x12s that clear everything underneath including the generator exhaust and work well. However, their is not clearence for a tire stop so someone has to tell me when to stop.
The 6 layers of 2x12 are 66", 56", 46", 36" 26" , and 18". To ease the transition I put a 1x4 11.5" long at the nose of each 2x12 "step" each layer is screwed together with several 3" deck screws.