Zach, maybe you can find some aluminum channel that will slide into the upright supports. Attach the crossbars to the new channel and slide into top of uprights. Drill a hole through the uprights and new channel for a keeper pin. Two positions, one down all the way, and one up high enough to allow the H16 to go underneath. The new channel would not really carry any load, just be there to retain the top rack when you slide it up.
P.S. Check to see if that aluminum used for the cat trax rack will fit inside the uprights... That is a standard aluminum extrusion used in shop tool setup. It is available from McMaster Carr, or maybe a local industrial tool supplier.
http://www.mcmaster.com/#t-slotted-extrusions/=mzc0cqMike
