I have two Losmandy Dovetail Plate Adapters
This is the description from the Losmandy website:
"Mounts to existing rings or telescope. For use with dovetail plate. Once mounted,no tools are needed to interchange equipment."
A picture of these dovetail plate adapters can be seen at:
The black anodized surface shows some signs of wear; functionally they are perfect.
Asking $45 plus shipping
In addition, if you could use a dovetail plate/base, I had one custom made to work with these. It is 9" long and 3" wide and 3/8" thick. It could be drilled to fit the mount you are using. Would be willing to include for an additional $15.
Thanks for looking