Well, when you try to make and fit them, you will understand at once the shape that is required.
It has to be oval to be inserted into the hole in the inside wall of the (double wall) rim. It needs to have a countersink to match the nipple head so it can rest at an angle that is matching the spoke angle. In the rim, washers can be made of any metal, but if you need washers for the spoke head to match the motor flange, those have to be small and soft so you can punch the spoke head into them.
Flange washers are not always necessary, for you may be lucky and have a perfect match. Rim washers are a must to build a strong motor wheel with a bicycle rim. Even the best MTB rims are not thick enough to stand very high spoke tension, and the nipple head will crack the rim sooner or later if the force is not spread around the nipple hole.
Motorcycle and moped rims, even some very thick single wall bicycle rims are strong enough and much easier to lace a motor in, but they are also very heavy. Performance and efficiency are greatly affected by wheel weight, mostly rim and tire since they are at the longest lever point from the axle, thus making the most resistance to acceleration and braking. There is a neat performance advantage using bicycle tires and light weight double wall MTB rims, that is well worth the trouble of building a strong light weight wheel.
If the bike frame can clear them, using wide rim and tires is also making a bicycle wheel much better, both for traction and resistance. On a wider rim, a tire contains more air volume and will require a lower PSI. The more air you have between the rim and an impact, the better the wheel can survive. The more rubber surface touching the ground, the smoother and safer the ride. The optimal ratio is a rim width that is 2/3 of the tire width, so a 2" rim with a 3" tire is a perfect combo for riding fast and hitting hard. After that, suspension tuning is your best investment to make your wheels survive extreme abuse.
