Well, there is such a surgery. It is relatively commonly (or at least, not rarely) performed on breeds at high risk of developing bloat - such as Great Danes.
However, boxers aren't such a breed. They're about 11th on the bloat-prone list, with risk that's a long, long way short of the leader. Which isn't to say it never happens, but it's not that common either. There are also environmental/lifestyle risk factors that can be managed to reduce the risk further.
I would personally be rather cautious about preventative surgery for something that isn't that much of a risk in the first place (different for dogs that have ever bloated, of course - as the risk of it happening again is very high - but that's not your case). This sort of stomach surgery isn't minor, after all, and you may well be in the situation that the risks of the surgery are far higher than the risk of developing bloat
I would also have reservations about stapling the stomach of a still-growing animal to it's still-growing abdominal wall... If you are really serious in considering this, then it may be a good idea to discuss the concept with owners of breeds that are at high risk of bloat, and for which this sort of surgery is rather more common.