Airtronics used to sell a mixer called a Vector Director that is very similar to the old Dodgson mixers, except it's made of nylon wheras Dodgson's were aluminum. I have one i could photograph for you if you want to try to scratchbuild one. (No, mine's not for sale).

I've mixed V-tails and elevons with the airtronics mixer, as well as the Dubro and sliding servo mixers. They all work well, the Airtronics mixer is the most adjustable. You might find one on eBay.

Cheers,
Dave