Two things (for starters) we need to know to get you a better answer:
1. Is the fuse a REQUIRED element in your project? Fuses are not something we typically use in the hobby. The power circuits we use are pre-planned so as to avoid overdrawing or overheating components. Even then, if one component fails (say, the motor), the remainder (ESC, servos, receiver) are needed to recover the aircraft, so a master fuse is NOT preferable. That's a quick way to lose a bird.
2. What kind of plane are you going to build? Style-wise, that is. A 15A power system is NOT easily going to get you into the 300W+ range. The quick math tells me that you need to operate on a minimum 20V (16-cell) system (300W = 20V * 15A) to make that happen, and that much weight will be a bugger to get airborne. And how much payload are you talking? The same airframe is not going to do both missions well. Even a 19A system on a 3S lipo with a 200W motor is just good enough to get a powered glider into the air with any decent speed. A wing may do better, but more info about what you had in mind, and the ACTUAL limitations/requirements of your project will help us throw you some options.
Yes, I've known that I'm "different" for some time now...