Now I would still like some protection for the front element. Maybe a step up ring from 67mm to 80/82 and then a clear filter would work?
I have the same lens -- the Schneider Super Angulon 90mm f/8 -- but use it on 4x5 only, so I can't say if a single 67mm filter (e.g., UV) will vignette when shooting 6x17. I do, however, also have the Fujinon SW 90mm for my Fuji GX617, which does not vignette on 6x17 with a single filter mounted (i.e., the dedicated 77 mm Fuji center filter).
The Fujinon lens has a larger image circle than the Schneider (250 mm vs 216 mm), which may explain why it doesn't vignette with one threaded filter. My guess, though, is that a single 67mm UV filter would be fine on the Schneider. Using a step-up ring (67-77mm) would give you even more clearance and room to breathe.
On the 90mm Schneider, I use the IIIb center filter, which is technically an 86mm filter with 67mm male threads on the lens side. For stacking additional filters (e.g., graduated ND filters, B&W contrast filters, etc.), I have the Lee FK100 Slip-On holder that slides directly onto the outer diameter of the IIIb center filter (no Lee dovetail adapter ring necessary). With the slip-on holder, you can use 4x4" or 4x6" filters on top of the center filter without vignetting. The Lee Slip-On holders are somewhat rare these days, but you can occasionally find them online. S.K. Grimes also makes their own version of a slip-on filter holder in any mounting diameter you want (I had them make one that fits the IVa center filter used on the Schneider 72mm SA-XL lens).
