Keep in mind what I said about checking my math, seriously.
Also, I'd think different strength lenses would give you different distance to the chromes, which would help keep the size (length) down (see Mr. Bill's point about distance).

You could do a mock-up with cheap parts to find what you need, then search for good materials.
Tape something together made of cardboard, using lenses from cheap drugstore reading glasses (in various magnifications), etc. just to get a feel for what may work best for you.
You could even use a pie tin for the reel