I bought my original ramps from the Guinea Pig Cage Store but for subsequent ramps, made them out of spare coro. The tricky part of working with coro is knowing which side to score the coro to allow fold properly. Essentially, you want to make a ramp in a rectangular shape, with the main part being wide enough for a piggy to comfortably use. The width of an adult's hand (so about 4") looks about right to me, with sides folding up . To attach the ramp to the loft, leave a tail on the ramp - the part that the piggy uses to walk on, that can be tucked under the loft's coro, and secured with either stick on Velcro or a two sided tape, etc.
The downside of lofts is accessing underneath. Our piggies like to run and hide when I try to pick them up, and trying to reach under a 2 x 2 loft would suck. Do you have room to OFFSET your loft, meaning that one portion of the loft hangs over/cantilevers off the end of the cage? If you do a google search on offset guinea pig cage loft, you will see examples.