I almost never use a Dremel tool on any spinner either, after almost ALWAYS having one grab in the manor that Brett describes, and I finally learned my lesson. On plastic, one of those key hole saw looking blades that you can get for the big red handle will get you the basic shape. I would sketch out a template that you want to opening to look like, and lay that out on the spinner, and use the leading edge of that layout, make it line up with a mark or line that bisects the spinner. Then put the leading edge of the template against the same line on the opposite side of the spinner, then cut away with the saw to get close. I usually just use a sharp knife to shave the plastic to the line or mark I am shooting for. Get it looking good but don't sand or file on it yet. repeat the process for the other side and get it to the line. See if your prop will fit into what you have so far, and then enlarge them if need be. If you are happy, now start the balancing and finishing work, if you have the proper balancing rig.
For aluminum, poke a series of holes close to the line, then trim out the big slug from there. Then use a file to start working things down towards the line and get it as close as you can before checking balance. I don't know how guys did it in the good old days before we had these neat new balancing rigs. If you are very careful with layout, and then the cutting and filing as well, I'll bet you can wind up pretty close to balanced. I don't recall ever seeing any kind of article in the old magazine on slotting a Froom or a Veco blank spinner. Veco spinner, I think, had a sort of "pilot " cut out that you could at least measure from. Froom spinners were just a blanc cone, I think, weren't they? I just never see any new in box Froom spinners to know for sure.
Type at you later,
Dan McEntee