After owning a couple of different harnesses I for one still like the ST-138 PRC 25/77 military radio backpack.I've got the RA presently and sold my old JP with the ST-138.The only thing with the RA is that it really puts all the weight on the shoulders which gets tiring after a short period.Something that distributes the weight is a lot more comfortable.
Off ebay.There are usually a couple on at any given time.I first got the idea for it off OB1's Jedi Acadamy site.The pack needs some stripping down,but it's perfect for a cheap price.
I also use the PRC25/77 harness, though I modified mine differently than the way OJA did it in their tutorial. The weight of the pack will still drag on your shoulders after multiple hours of wearing it, but the system itself isn't uncomfortable to wear.