I guess it's a bit late now, but I bought a handful of 2x10s and screwed them together, with 45 degree cuts with the circular saw to provide a bit of an easier climb to the next level. The topmost platform has a 2x3 at the end as a 'stopper', and each portion of the ramp has a roughly 18" flat section before the start of the 45 up to the next section. Heavy, but I'm confident in their holding ability for a while.
Mind if I ask what ramps you ordered? I have a pair of the plastic ones, but I get kind of antsy about them over time as who knows what plastic will do when it lives in a garage with a wide array of temperature swings.