Umm, I'm not sure about their size, I have them but happen to not have any rulers laying about. But I do know they'll hold more weight than any split rings you'll find.
But you can get heavy duty split rings at Feuershow, but you might want to get the tool to split them (also on Feuershow) as they're a major pain in the ass.
I don't know what rings you can weld, or how to go about doing it, sorry. I'm not sure what tools you have, but I don't think making them will save you much money at all. As I've found from looking at materials to make things, most things I'd barely save any money at all. Like with fans, I'd save like 20 bucks, so I just decided if I get them, I'd fork up some more for professionally made ones, then I decided later I want double staff anyways.
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The only luck is bad luck.
Shut up before I stall my poi up your ass