Actually just got done with one of these today, following dejoha's instructions. Spent about ten minutes with it on the Hennessy after putting it together (too hot for any longer; it was 90 degrees out today), and I haven't had any issues with ridges under me.
I used mason's line for the fixed line running through the PLUQ and standard 1/8" flat elastic cord (not shock cord; the kind of elastic that gets put inside of cuffs to keep them taut) for the cinch ends and tie-out points.
The only thing I can think of to explain the ridge feeling you're having is tightening it too much. It's possible that it's something else, but that's all I can really think of. I've yet to try mine in cool weather, though, so it's possible that I'm hanging it too loose for heating purposes (sure didn't feel that way today, though).
Hopefully, things'll cool down here soon again so I can try an overnighter with it in the yard. I'll let you know as soon as I've done so...