Predator uses M16x1.5 threads for their weight bolts. Just buy an assortment of different lengths of set or grub screws, get a long allen wrench of the right size ( think this will be 10 mm, but check. At least it isn't the two pin thingy they want to sell you for an exorbitant price), weigh them on a good kitchen scale and go at it. I do the same thing with the 1/2-13 bolts that fit McDermott, Joss, Pechauer, Athena, Schmelke and probably a bunch more that I just haven't come across yet. Stack them in any combination you want, but make sure they are all tight so don't rattle. If you want them further back for balance, drop a piece of wooden dowel or a wad of cardboard to the bottom of the core and tighten it down with the first bolt. Matt has it right. This isn't rocket science by any means.
They could have probably gone with a coarser thread that is a little more available and cheaper, but at least they went with a standard metric size. Most new machinery in the US is now metric, so in the international fastener market over 90% is now metric, with SAE production going down year by year. A few places in China still produce SAE sizes, but it appears some of them went under during Covid, so curiously the SAE sizes are now showing up as more expensive than metric, which will just hasten their demise. The remaining manufacturers would probably be glad to not make these for a dwindling market and just tool everything for metric instead.