Hello, over my years of slinging I have noticed that the most common ammo you can get commercially and the easiest to mass produce on your own are projectiles that are round. I.E golf balls, tennis balls, and clay balls that you can round with your hands. I have a good number of slings however most aren't adequate to use the most roundest projectiles. I understand netted pouch slings exists but I don't have the skill to make them, is there another type of pouch that would hold them without them slipping out?

Joined split pouch slings work very well for spheres. See my post "easy full leather sling" for reference.

Do the seat belt material pouches work as well with spherical objects as the leather ones?   

Here's my Butterfly Net pouch design.  I used multiple strands of bamboo yarn chain sennit braided to appropriate lengths. Works great with golf and tennis balls, mahogany nuts, unripe blown-down baby mangos, and similar roundish objects. 

The pattern is in the PDF Leather Pouch Templates thread --page 7 -- here under General Discussion...  You can, of course make the openings larger, should you choose.

I'd say there are two criteria to consider with spherical regular ammo; (1) you need a pouch that will hold it securely, and (2) you probably want to achieve as clean a release as possible since lightweight spheres are subject to quite strong aerodynamic effects due to spin. Heavy ammo this is much less of an issue.

Split pouches can fail on (1). You could make a smaller split pouch specifically sized for a sphere but it wouldn't be that versatile. It can attain good clean release (2) though. Overall I consider split pouches that are built right to be very secure, however spherical ammo is the exception which requires modification of the design. I would say specifically in the case of tennis balls however that they are rough and large enough that a split pouch is unlikely to have a problem with them and may even be optimal.

Solid pouches are a natural and decent choice for the task, satisfying (1) quite comfortably. However depending on design and your own technique you can impart a lot of spin and coming a cropper on (2). I will add that cupping is probably ideal with solid pouches in this use case, since spheres will not sit very comfortably on a flat surface.

My favoured choice would probably be some sort of semi-split design like an aussie pouch. These have some of the cleanest releases of any sling in my experience and are also very safe and versatile with lots of ammo types.