I made this a while back on PGPOG and forgot about it till recently, just throwing it up here since PGPOG isn't as active nowadays.
I would suggest using rubber or soft plastic nowadays, or even try the fingers concept like those on full size loaders. Making this thing made me felt like one of those prisoners who creates makeshift weapons do to limited resources. Good times I say!
I took a piece of 3/4 Inner diameter copper tube, cut it lengthwise so it forms a C type of shape and cut plastic strips, bent them over the end (so balls go in, cant come out) and taped them there. Cost me .25 (pipe piece) + thin plastic from toploader baseball card protector (.05) = .30 cents. Ugly as sin, but it works, I don't have $28 for a fancy speedloader at the moment.
I didn't measure with any rulers or anything, i just took unmeasured cuts at it and used common sense, always cut so you rather get an extremely tight fit rather then a loose fit, so if it's too tight, you can bend it a bit till it fits