Originally Posted by KarTek

If it was me doing this, I'd find the closest size load extender to what you need, cut a section out of the middle of the tubes equal to the size you need to shrink the thing down to fit. Then, find some tubing that just fits inside the tubes you just cut. Cut a few 6" pieces, slide them into the cut off ends and then, drill and run some bolts through both pieces to create splices.

You may be able to use JB Weld to splice the pieces without using bolts but if something slid against it and broke the bond, you'd have your load coming right out the back onto the road.

This one appears to be fully adjustable and I think it's even pictured on a Sonoma/S-10.

http://store.summitracing.com/partdetail.asp?autofilter=1&part=TLM%2DBX4004%2D02&N=700+4294846010+115&autoview=sku