Simple Adjustable Fairings is a fairly mature mod that allows fully modeled, switchable length fairings without much fuss. It would be great to see them for NFLV parts. You basically make the nosecone (which doesn't have to be symmetrical, you can have a nose cap as part of one of the halves), the walls and, for hammerhead fairings, the boattail. While there are old KW fairings available for up to 5m, 7.5m size is unique. With SAF, you can have highly detailed walls, properly rounded noses and bases. Also, the KW fairings were nice in their time, but they now show their age, so a replacement for them would be greatly appreciated, if you feel like making one.

With B9PS, you can have multiple sizes on a single base. I would propose the following sizes: same diameter as the base (straight), just big enough to comfortably hold parts the same diameter as the base (small hammerhead) and just big enough to comfortably hold a part one size bigger than the base (large hammerhead). For example, 7.5m base would have fairings 7.5m, ~8.5m and ~11m in diameter. The latter would be able to fit the huge spherical tanks and the 5m SSPX centrifuge. Same principle for every other size you care to make. These would cover most use cases.