Merc, VP, Marine power, Indmar all mount relays on motors, and for many years all where using the same regular automotive type (4 blade square), or at least that I have found. So don't understand the concern

On race cars (modified dirt cars) where the engine is bolted to the frame without rubber mounts, we found that they had a short life until we mounted them on rubber mounts, not saying they will fail but vibration/heat does shorten the life. The ones I have seen on boat motors are typically in a rubber holder except for the starter slave solenoid.