Here's how I mounted a piece of Unistrut. The hex-head cap screws rusted over night because I stripped all the oil off of them and used purple Loctite (low strength) for the threads.
Some angle brackets that were just about perfect for this application, 6mmx12mm long cap screws, some 1" aluminum and the existing threaded inserts that came with the rack:

I had a couple vinyl caps laying around, so I threw them on:

Before drilling the two holes in the 1" aluminum:

Angle bracket and steel cap screw #s from Home Depot. Stainless, 6mm screws are on the way:

Back to the ladder mount bike rack, the build write-up is
HERE.http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27582581.cfm