Can anyone recommend any accurate sei whale figures? If not, how about a similar species that could be easily modified?
None that I'm aware of... but the smallish (146 mm) Maia & Borges Blue Whale has a narrow, v-shaped rostrum instead of the rather u-shaped typical for
B. musculus:


With a modification of the fin and a repaint this could be turned into a decent 1/100ish Sei Whale quite easily, I've already thought about modifying mine into one.

Of course, the number of gular furrows does not match (neither for a Blue nor for a Sei Whale) but this is almost never the case for rorqual figures.