I'm guessing that to build a beam expander that would thread onto your WL, with the lens mounted correctly would be more ( much ) than the 149.00 that WL would charge you. Maybe I'm wrong but I'm guessing not ?? But best wishes.
Well yes, but the same could be applied making literally anything...
What Bob is getting at is that you can likely just buy what you need for much cheaper than getting it custom made. So unless you need different specs from what WL offers, you'd be better just buying their expander.