I found a video by Lee Harrington on Kink Academy a while back (I don’t subscribe any more so no idea if it’s still there) that shows exactly the kind of strap-on dildo harness you’re describing – totally made of rope and fairly simple to put together. I’ve tried it several times, it’s a cinch (literally 5 minutes or so once you get the hang of it), totally adjustable on your body, and able to accomodate any size dildo. I’ve put it on myself, and had my sub do it for me.

Basically, you tie around the waist first, pass rope between your legs and back again to the front, and begin a series of hitches down the pubis and around the dildo. Pass the rope to the back, separate the lines and pass to the front again, tying off at the waist. It will leave your crotch and ass exposed for play, and the best part is when you’re done you simply untie at the waist and the whole thing falls right off for cleaning later.