I plan on doing the same with wood... I have the metal ones that are punched out, and I don't have a paved driveway. My old '02 Impala would sit stable on them, but front would be 1" in the ground.... I don't trust the Ram I just got to sit on them very well... heavier vehicle and wider tires.
I was also going to use 2x12's... but debating is 12" is wide enough or maybe 16" would be better.
If I went 16" wide... I'd probably use 3/4" plywood. Be a lot of layers but smaller steps and easier to drive up when working on the mini van. Figured 12 layers, be a 9" lift, glue and clamp together. Heavy, but solid and won't sink. Otherwise if 12" wide is fine (need to measure what I have) then 2x12, 5 layers.
Seen a few boxes made with plywood, open cavities inside... really don't trust that idea. Debated the plastic type ramps, but open cavities in the bottom mean sinking again.
Also playing with the idea of an 18"-24" deep mechanics pit beside the garage and 24x24x2 patio stones. Not too deep, but enough to get under for fluid changes, exhaust work etc.