I use two bent grids, one bent at a ninety-degree angle, and one sorta-bent, maybe at one side it is thirty degrees and at the other end, it is twenty (from the normal, of course, I guess the real angles would be 150 and 160, respectively. Very obtuse.) Anyway, it's hard to describe, but I hook the two together, with the 90 degree grid on top attached to the top level (with one side of the grid acting as a guard rail so they don't fall off the side), and the other grid hooked up to the first one with the sharper angle first. It works out so that the bottom grid forms more of a gently-sloping base/short platform. It's great because it gently slopes off to the side (into the middle of the cage), but if the pig were to go straight, there is a 3-4 inch drop off--basically, if they want to run in circles on the outside of their cage, they just run underneath the ramp and come out under this side. The whole thing is covered by a throw rug I bought on clearance from walmart, which makes it stable and gives them traction. It drapes over the edge where they run under.
I'm sorry if my description makes it hard to understand!