I ran out of cube sides, so I left the ramp resting on top of a house we bought for the pigs. Lo and behold, they can (and seem to really enjoy) jump up on top of the house and get up the ramp. The towel is tucked in under the roof to give some traction, as they have problems with bare wood. The house is about the same height as a cinder block, which would also work well.
The ramp is just a cube side folded into 3rds with leftover bits of coroplast forming the bottom (with crosspieces for traction). I'm worried it might be rough on their feet, so I'm going to get some carpet scraps to fasten down to the ramp bottom.