I have the 1/4" version of this going spare (SKU 030-14); it's new-old-stock in the original Manfrotto bag... but I see you already have one of those. I have some 1/4" to 1/8" aluminium adapters if that would be of use. You won't get as good grip on the plate though of course.
apologies for the delay, here's a shot showing both sides of them:
http://www.brodie-tyrrell.org/crap/dsc18026_640.jpg
probably not something you'd want to use though because the gripping surface on the top is so small. if what you're mounting on there has a 3/8" thread I guess it's pretty heavy and needs the grip area of the hex plate. Let me know if you're still interested though.