I put this on RBR too, but it's such an easy fix I might as well share it. In an overfilled truck moment of inspiration, I took the 3 fork mounts off my trusty 2x6, ripped the sides off an unused Yakima tray, milled a dado into the lumber, and presto! Cheapo adjustable board.
I drilled out some new holes in the mounts, threw in some carriage bolts, and used Yakima wingnuts. All in all, it was an hour of work I should have done years ago. No idea why it took so long to percolate into my brain.
Mtbr's 2016 Winter Biking GearReviews and Roundups
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Thread: Homemade Adjustable Glider