Dan,
You do not need any special permits to make motor casings. You may be required by your state law to purchase a vendors license if you are going to sell them to the public.
What Jerry was trying to say was that if you plan on making motor casings for commercial reloads that you can buy, such as those from Aerotech , AMW, Cesaroni, Ellis Mountain, etc..., you should check with each manufacturer and see if they can have your casings certified to use with their reloads. Some of the mfgr's may not have a problem with this and others may say no, but it doesn't hurt to ask. Once your casings are built to their specs and certified to be used with their reloads (this process includes manufacurer's cert and then official certification by either TMT or NAR), then the casings/RMS can be used with the respective commercial motors at all NAR or TRA Sport launch events. Mind you, this is only for certified reloads. Also, please note that with the exception of Aerotech, all the other manufacturers exclusively make casings themselves for their reloads and may not welcome the competition. Moreover, you may run into copyright problems with such vendors if you try to produce a similar RMS system for their reloads. you may be able to license production through them, but I have no idea what costs they would require.
Now, if you wish to avoid all of that and feel your products would do well in the Experimental market, you can simply manufacture your casings and sell them without certification for EX users. There are only a few companies that currently manufacture casings for this arena, so it may be a good way to start and allow you to get a feel for the market. These RMS Casings could be used at all Amateur Events, Personal Launches and at Tripoli EX events.
I hope this cleared it up for you a bit.
Carl