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?

I've run into the same problem with split pouch slings.  A split pouch can definitely hold a spherical projectile, but it has to be just the right size.

I would recommend a leather pouch that has a cup to it.  If you checked out this page, https://slinging.org/forum/YaBB.pl?num=1344443925, there is a really good design if you scroll down near the bottom.  CA called it a 'butterfly'.

That is basically the same designed used for this sling: https://youtu.be/z17OLpWALhk.  The projectile in this video is a 2.5 inch diameter ice ball.  So almost the same size as a tennis ball.

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

What Joe said.

The butterfly is a great place to start.
If you attach it to the cords with loops as I recommend, you can make them out if even fairly thin material.
Canvas, denim, double denim etc

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

Sure.  The material isn't as important as the shape.  You want a pouch shape that the spherical projectile can sit in and be supported so it doesn't roll out.

Along with the butterfly, the Aussie pouch design is a great one for spheres.  The sling in this image is not specifically the material or dimensions of an actual 'Aussie sling', but the idea is exactly the same.  The two pieces of leather (or other material) can be made to whatever size you need.

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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. 

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   A sling with a deeply cupped pouch like this modified pj does the job plus it's easy to make:

http://slinging.org/forum/YaBB.pl?num=1338672380/0#0

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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.