I have 2 of these type of simple bullet sizers in .50 caliber that were purchased on eBay. Both came with a hand written label attached that states "CVA", ".502" or ".508".
In essence, they are much like a heavy duty shotgun choke tube that gently steps down in dimension to squeeze the bullet to the desired diameter as it's pushed through the chamber. There's also a steel rammer to ram the bullet through with.
Short of finding a sizing die of some sort with the proper dimension, the only way that I can think of to duplicate such a device would be to take a short section of a rifle barrel and then to have someone hone it to the proper dimension required. Most muzzle loading barrel makers/gun builders probably have surplus short sections of barrel and could do the honing, or even an inexpensive pistol barrel from some parts outfit like Numrich might not cost too much if he knows someone who could do the honing.
Maybe there's a way to take each bullet down individually by using hand methods? Other's have mentioned doing it for revolver bullet experimentation. I suppose a dremel tool with a wool, bullet shaped buffing tip attached, and with an application of a fine polishing grit might be able to polish the bullet to fit. Or using some kind of buffing wheel or super fine sand paper?
But it would be a tedious task and subject to some dimensional trial and error.
How about a rock tumbler (or rock polishing drum)? They sell those at hobby stores to polish rocks along with the right polishing medium. Come to think of it, some people put their home cast lead balls in a bucket filled with fine sand and drive around with it rolling around in their trunk until the balls are nice and smooth and the sprues are gone. But then they're used with patches to make up for any diameter loss.
That's all I could come up with for now.