It should snap to a wall just like a trophy mount

It' not the green for it is flashing like it wants to snap, but I can't get it to snap for some reason.

Rafts are a bit buggy with placement, and sometimes want to snap to the back side of the wall. The best advice i have is to just keep trying to move it around the spot until you can get it to snap.

This is an old post but I ran into the same issue and wanted to post the work-a-round for others also running into it. MacGyver is right in that it only wants to snap to the back sides of walls. The trouble is you want to put the figurehead in its proper place, which will be on an exterior wall. I worked around it by placing the figurehead from the inside of my ship; you'll see it during placement flickering between facing forwards and backwards so keep trying until its attached on the inside of your ship but facing forwards.