The 18350 version of the Convoy S2+ is the cheapest option I’ve heard of, to minimise your losses if it doesn’t work out, but the S2+ is still a good quality light.
Has anyone tried using a slightly thicker o-ring in their flashlight for a tighter seal? Say, 2mm thickness instead of 1.5mm, for example? I imagine it would be subject to greater wear - assuming the light screws shut at all - but grease might help (grease improves waterproofing anyway) and o-rings are cheap.
Another idea that comes to mind is that some flashlights like the S2+ unscrew at both ends, but other flashlights only unscrew at one end. That would reduce the number of potential entry points for water if the S2+ didn’t work out. Alternatively, you could try gluing the S2+ head on with something like silicone sealant to achieve the same outcome.