We use the yellow plastic "lego blocks" from CW like in the 1st photo (not ours). Have 3 packs of them. They are lightweight and take up little space in a pass-through compartment. They are an inch thick and you just stack them up as needed. For tandem axles you just stretch out the buildup. Sometimes I've been 1/2 way between another layer and have simply placed an extra layer under one wheel only.
The only thing I don't like about the plastic blocks is that they can get damaged on sharp rocks on a site. Have had to toss maybe 3 of them in over 4 years so not a big deal.
The lego blocks also are useful for other things too like under jacks, leveling a picnic table or BBQ and even making up a step at the bottom of your entry stairs if on a really sloping site.
Personally, having to use bulky cumbersome chunks of wood and store them somewhere would bother me. Another option is Anderson levelers as in the 2nd photo but they're awfully expensive for what they are plus are also susceptible to damage from sharp stones.
