Take a standard inner baffle diffuser and get someone to sew a circle of diffusion in the middle. Play around with different sizes & thicknesses (use pins or something) if you want to dial it in.

When I need something like this done (custom scrim-jim fabrics, etc) I go to the fabric store - many keep a binder of business cards from little old ladies who sew. Anyone who makes draperies & curtains (most of 'em do) can make you all sorts of stuff. I had a blackout curtain made for a large studio I had - 15' x 18'. I supplied the fabric. Lady charged me twenty bucks, all I needed was a grommet tool and a sack of grommets.

By the way, I have a 6' octabox for my speedotron gear - it fires towards the face, just like any softbox. The only benefit is it's much shallower than a similar rectangle. Can't recall the maker, but it is handy. I like it without the face and just the internal baffle for a look that's soft yet harsh...