Maybe I could do something more precise with some 5/16" bearings, but this is a pretty good fit for a lightweight handle.
Actually it was 3/8" bore bearings that were large enough to fit the 1/4" drive piece, which has a little over 8mm thick shaft, and I made up the difference with tape around the extension underneath those bearings. 1/2" bore would fit over the average 3/8" extension, but I got away with 12mm over one layer of electrical tape on that particular one (it's old, and I don't know the brand, but another brand was more than 12mm). Then I put as many spring clips as would fit to the end of each extension (had an assortment that I rarely used), and wouldn't want to stack the bearings up much farther than those on 2 & 3" extensions, or they'd get pretty heavy.
The other kind, with a hollow anodized aluminum grip, has only two thinner bearings inside, with a groove for the spring clip in front, and the extension shaft is slightly thicker where the bores fit on there. I was approximating that with the tape for these shorter ones, and couldn't find this kind for 1/4" drive anyway. Ball bearings don't necessarily free spin faster than a bushing or plain bearing (like the socket I have on the other), because their casings give them some resistance (which is noticeably overcome by heavier handles or powered drives). They're fine for spinning smoothly by hand, just not like a top. I like that they have a larger diameter to grip, and don't wobble at all on there (I've wrapped sports tape around them since).
You can see some marking on the 3/8" extension end, where I tried using an impact socket as a bushing initially (because the inner bore was large enough, which varies between sizes), so obviously those aren't the best for this. Chrome sockets are better, but I'll have to keep an eye on that one too. Maybe I'll find something hollow to fit over two bearings without getting into machining such parts myself. As it is, I like this one fitting over the whole extension and matching the ratchet diameter, for an optimized grip length, so I might get away with leaving it at that, for occasional use (or at least until I wear one extension down to be sure, it feels pretty smooth for the time being, and the tiny handle probably helps to reduce friction). Ball bearings, on the other hand, don't rotate in contact with the other part, so I guess they'd last much longer with heavier handles.
What's cool about the little handle (as I was getting at before) is that when fully extended to the side, it is the same length as the 1/2" drive tee that is spun from the middle, while this lighter one feels just as balanced either way. Also, the sliding 3/8" handle can be positioned somewhat off center with the magnets on there, without feeling lopsided and falling on its own, so they're all equivalent in leverage that way, besides the non-sliding one (which is otherwise easier to follow around with its ball-shaped ends). Well, when I got the micro handle, at first I was thinking it was like a keychain tool charm type thing. Now it's more like Mighty Mouse, methinks (not that I'd stand on it, or an ant either, if I could help it, just that those are said to be much more powerful than their own weight).