These are all good ideas and all work well. I have used plastic as well as wood. Both worked well but wood held up better in cold temps where plastic cracked.
What I have found to make things easier is to install a set of levels inside the cab in order to get it as level as possible without anything under the wheels.
Go somewhere close to level to initially setup the rig. Once you get the fridge level (by using a bubble level inside the freezer) then install (screw them into place) a set of small rv levels inside the cab area so you can do a visual check as you pull into a site. I placed one by the cab radio and one above the passenger door to give me an easy visual.
Many times I can get it very close to level N-S-E-W without too much leveling by using blocks. It comes in handy when stopping for quick roadside stops or shopping stops.