Nope it does not have to be strong at all as your flap/door is Vertical all the time,think of a box side on and draw a line inside it from corner to corner Bott left to top right,there is your slope.As for servo strength it is well powerful enough as they are used in every boat to control the rudder,if they are powerfull enough to do that then they do a door/flap with ease.The main trouble with what you want to do has major problems believ you ME.
The best bait boats are the ones with either a compartment that opens to the side of the boat or underneath.I have built many a bait boat Boilies arnt a problem as they are round and roll down a slope with ease,chopped ones,hemp,particles of any description are also a nightmare,even built one that had a hindge at the rear and had springs underneath and operated by servo,it was great for partices as it would scatter them easily but a nightmare with boilies as it scattered them everywhere !!!!!!!!!!.
Even if you built a hopper/rear box particels WILL stick to it with ease,BUT if the hopper/box is underneath like the Angling Technics range.
Dave