hmmm. . . I take it you didn't get the derilin motor mounts with the knuckle quad frame? they should have allowed you to mount the motor upside down and connect the collet prop adapter on the long side of the shaft.
If you're handy with tinsnips and drilling holes, you could *make* a similar mount to screw on to the end of the boom with locally available materials -- I'm thinking a small piece of thin/sturdy sheet metal might be do-able, but that'll depend on your skill with the material
The prop-saver adapter is less than ideal, but for what you'll be swinging, it should be passable -- problem with quads is the agility is entirely dependent on how responsive the props are to throttle changes. Even a tight prop adapter will have some slippage when changing speed -- enough to notice, but won't ruin the flight. You'll still have to find a way to deal with that motor shaft sticking out the back -- drilling a long hole in the boom is less than ideal -- get the motor slightly off level and it'll rub against the boom and cost you power and service life.
If you choose to go this route, make sure the band is *TIGHT*. don't use the rubber O-rings, they're usually too loose. try small rubber bands, doubled /tripled/quadrupled . . . whatever it takes to hold it on hard. It should take some doing to get it on -- if it's easy for you to put on, it's easy for the motor to sling off.