I found the NiteRider Stinger okay, although I think it turned itself off a bit when it got water in (so not brilliantly sealed) and the watch type batteries aren't great as you can't get rechargeable ones. Latterly, it seems to have been behaving itself. I lost it recently, so Santa brought me a Topeak Redlite "Aero". The idea of an aero rear light is hilarious, but it does mean you won't catch shorts on it.
Comes with two sizes of rubber strap, and seems secure. Takes a AAA battery so I can just use rechargeable AAA batteries for minimal cost.
Not used it probably MTBing yet, but done lots of commuting miles (mostly with mudguards) and some puddly CX miles. No complaints yet.
Like you, I was looking for a rubber design for easy swapping between bikes.