Ive used various methods over quite a few years. I always had a system of tetherting it over the A frame as it was deployed, tried and tested in all weathers and worked perfectly. The issue really is about position for A: weight distrubution B: security so it wont rattle free whilst the boat bounces about and C: protection from being swamped by inboard water.
One of my last boats (5.8m) had it installed up in the bow, just aft of the bow locker. Bit of a pain in the ass with deck space getting used, but the power head was well sealed, and I'd used some flat foam fenders and cut out a profile for it to sit in, then bonded them together, so it had a perfectly snug fit then I installed 3 U bolts on the deck for it to be lashed down with ratchet straps, yet easily released. The weight up front made this hull really work well
. Any water taken in pretty much goes aft fairly rapidly, and the bilge pumps get rid, or if it was a likely factor the trunks would be down anyway, and I never had any problems, the motor was a 6, 4 stroke.
Correct support was essential in this regard, because those staps could put a lot of 'pull' on the motor, and over doing it could lead to a whole new world of hurt. Ive had them standing upright bolted th jockey seat backs, lashed at the stern etc, but the method described above was probably the best on that occasion