The best method of all (I think) would require machining from a solid length of a suitable resilient material in round bar form (ideally, a urethane elastomer) a blind-ended hollow cylindrical form, with a reverse of the hose barb machined into the internal bore, and then force that axially over the barb, and clamp it.
Lacking access to a lathe and a rather exotic material, you might consider trying to find a suitable close-fitting solid internal plug
to fit the bore, and bond that in place (having cleaned out the bore of the thru-hull if necessary) and also check with hydraulic suppliers for a suitably sized moulded plastic port protector fitting - for some reason, these are often red (similar to the first idea, but not custom made, and thinner/ not intended to be leakproof)
and clamp this onto the barb as above, as a second line of defence, helping to prevent the plug
from being forced through to the inside.
At a pinch, the second purpose could be served by two (or more) rectangular strips of strong material (even heavy sailcloth would work) laid like a '+' over the 'o' of the thruhull, then folded down the barbed portion, contacting it, full-length, in four places, to be held by the hoseclamps.