crappy tire had some quad accessories and I remember seeing a big clamp type unit for carrying awkward longer stuff,, can't remember the name of the product though.
There's always the option of building a carrier. I have one on a bike built out of 2 x 8. One length lays across the rack secured by u bolts and then two uprights with notches on the top to hold the auger flighting in two places. A couple of bungee straps are held by anchor points screwed to the 2 x 8. The uprights are braced with angle pieces. Not the purtiest of things but works well and costs basically nothing.
There's always the option of building a carrier. I have one on a bike built out of 2 x 8. One length lays across the rack secured by u bolts and then two uprights with notches on the top to hold the auger flighting in two places. A couple of bungee straps are held by anchor points screwed to the 2 x 8. The uprights are braced with angle pieces. Not the purtiest of things but works well and costs basically nothing.