I, too, am a cheap owner of a Rhino Rack Pioneer Platform rack. I was looking for a way to mount my awning, traction boards, and cargo carrier without paying too much. Plus, I was looking for a project.
I bought 3/8" thick by 3/4" wide aluminum stock from McMaster Carr (Part # 8975K615) in 1' lengths and cut them into four 3" long pieces. I then drilled and tapped the pieces to 5/16-24 and powder-coated them black. I can slide them into the channels on the Rhino Rack and use them to secure my accessories.
McMaster Carr sells a variety of eye bolts that would work well or you can see what your friendly local Hardware Store has.
Let me know if you want a cheap idea for mounting the Rhino Rack to the top of your 4Runner. I already did that and bought the shirt.