Made some for mine:
2 - 2x6s, can be 24-36" long, side by side with a couple 2x4s underneath holding them together.
One 2x4 is at the end, the other is a few inches from the other end.
You can do it with 1 2x8, too.
To get on them, I use another length of 2x8 wedged underneath, into the recess that the offset 2x4 leaves.
Easy to drive onto, unlike the store-bought ramps that push away as you try to drive up on them.
Sorry no photos, they're packed for next week's move.