For clear OHP film, I've tried paper under the film, and that's worked, though as it happened, the film that I tried (not Pictorico) wasn't suited to inkjet.
The B9180 seems to work okay for me without a sheet of paper underneath using Pictorico Premium OHP Transparency Film, which is slightly white. This adds some extra density to the neg, but it doesn't seem to cause any problem when printing.
I tried cutting it down, but it seemed too fiddly, so I just print the whole 8.5x11" sheet. I usually want an 8x10" or larger neg anyway for contact printing. I tried enlarging a 4x5" neg from the B9180, and I didn't think the quality was good enough.