No I would not attach anything to the barrel either, as it will throw of the vibration/harmonics of the barrel. You didn't mention scope so not sure if you are trying to put this on a glass scope or thermal. If glass, they make scope rings with picatinny rails on the top. Since, this is on a night vision page, I am going to assume it isn't a glass scope.
The easiest would be to put a picatinny rail on the side of your upper. If you have keymod or m-loc, the answer is pretty easy. There are quite a few picatinny adapters for both. This is a minute or less install. If you don't, and if it is more of a tube type metal handguard, usually you can find some type of bolt system to attach a picatinny rail to it. If you don't feel comfortable, I would assume a local gunsmith could figure out some way to attach a rail to the side of most AR handguards. If you need more space to make room for the rangefinder once the adapter is on, you can always add a 1" riser and/or 90 degree adapters depending on the orientation needed.
