The Fein is a fine tool (:)) for many things - primarily those things that no other tool will do. But it is very slow at everything it does, and not necessarily that neat either. I don't know what you are dealing with in terms of space, access, material, etc. But I would consider making a 1/4" plywood template that encloses the shape you want to cut, plus a margin equal to the radius of the Rotozip base (minus the radius of the cutter, to be exact) and fasten it temporarily in place (possibly with hot melt glue). Then run the Rotozip around using it as an external fence. The result will be neater, far faster, and require less access and visibility.

The Multimaster is unique in its ability to do things no other tool can do - but when another tool can do the job, the Multimaster is always my second choice.