Ken Willard designed a model with a rhomboid wing in the early 70's. It's one of the RCM plans. Don't remember the model name or exact year/month, but a search of the RCM plans should find it.

Edit: I tried posting the links, but they didn't post correctly, and would not go to the correct page. Just select the 'Sport' category and then do a search for Ken Willard. You'll find the two planes.