From my experience the density/weight of the rubber horse stall mats is much more than wood. For the same size my guesstimate is the stall mat may be twice or three times the weight of wood. Maybe there is newer stall mat technology, but I would be worried about compressibility.
I have not found anything better than wood for levelling, (other than the 6 point level up system). Like others I cut the edges to 45 degrees. I carry one 2x8, the length of the tandem wheels, and two sets of stackable short boards (under each wheel) to build height on top of the 2x8 if I need more than one or two board heights. I carry a rechargeable drill and 3 inch wood screws to attach and hold the smaller boards to the larger 2x8.
Brian