from the description the prop and the spinner back plate doesn't have enough bite to grip each other as several posters have astutely pointed out.
don't put any sandpaper between the drive washer at(the engine) and a spinner back plate whatever type it is,alum.or plastic, the dust and grit can be harmfull in several ways beyond flattening the groves in the metal.

sandpaper discs between the back plate and prop is perfectly fine for slippage,which from your description is what is happening,also to crank that nut down tightly,part of the problem with plastic spinners is they squeeze out and get weak and can fly apart when tourqed down tightly,which on a 4st is important, on a saito100 the suggestions on getting a metal spinner is a good one, there much safer for 4st engines for a number of reasons, I don't use anything but metal spinners on all my 4st engines.

also something to consider is when you cut the slots in the spinner you might have thrown the spinner out of balance causing small amounts of vibration the plastic being much more flexable than metal allows the prop to slip no matter how tight you crank the nut down,just one guess since I havent seen your slotting job,but it doesn't take much.
its one reason why Tru-Turn,and Dave brown, ask what prop your using so they can slot your spinner accurately at the factory,if you get one from Tower the slots are over large to accept just about any size you may have.

also I have several, and like tru-turn spinners, but there the most expensive on the market and having to shell out 60-70+$ for a spinner is sometimes hard to swallow,there are several less expensive brands out there that offer many of the most popular shapes,such as Dave Brown products http://dbproducts.com/ which offer many popular types at half the price of T-T.
or another company that I have gotten spinners from is Racing Devil Hobbies http://www.racingdevil.com/product.php?category_id=105 which for the price are very nice quality and highly polished,they also come with a adaptor kit with the spinner included.