This is one of the setups I'm using. The flash just barely covers this 75mm lens and I'd like to be able to use the 65mm lens (28mm equivalent). Spec sheet on the unit shows coverage of 60 degrees.
I would just buy a sheet of drafting vellum to make a diffuser for this flash, and put them on with Scotch tape. Just cut out pieces to the right shape, which you can trace from the flash head itself. Stack them if one sheet is not enough. Then test to see how many stops you need to add when using the diffuser. If you wanted to get fancy, you could even make a solid frame to keep the diffuser rigid and in good condition. Some hardwood laminate (maple or ebony, maybe; try a luthiery supplier or local hardwood store), an X-Acto knife, and white glue would be a very easy way to make the frame "sandwich."
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