This thing is superseded by my Y-Carriage_for_20x20_X-Rail_Version-2
by printingSome July 24, 2021, https://www.thingiverse.com/thing:4915787,
which is much simpler and, I hope, better.

These are remixes of "3-wheeled Y guide (Re-D-Bot)", www.thingiverse.com/thing:1930200, by NickRimmer for use with a 20x20 V-Slot X-rail
Like for instance in my "X-Carriage, Linear, Modular, 20x20 V-Slot", www.thingiverse.com/thing:3350550.

For compatibility reasons I have imitated NickRimmer's version for the major part, so one can exchange them without any extra parts.

This goes for the
Y-Carriage-right_for_20x20_X-Rail.stl and
Y-Carriage-left_for_20x20_X-Rail.stl

but also for the other version:
Y-Carriage-right_for_20x20_X-Rail_and_Idlers.stl and
Y-Carriage-left_for_20x20_X-Rail_and_Idlers.stl

You can use NickRimmer's Y-Carriage-3-Wheels-2_x2.stl (green in images), but also my remix Y-Carriage-2 (just a couple of mm thicker at the bottom, because the former one didn't survive all my experimenting and cracked there, and ø3mm holes at the top, which you can make ø5mm if you want to use M5 bolts at the top too.

The latter ones can also be used with GT2 idler pulleys 20 without teeth, ø3mm bore belt width 6mm (20T W6 B3 in short), and for the purists among you "with teeth" at the back.

Edit (21 aug 19): well, those "teeth", I don't know. As I found out, experimenting with my Bed Hoist, those teeth don't always do much good. It's perfectly logical to use them here, but there it was a bad solution in one place. So I replaced the teethed idlers here too with smooth ones. It didn't make much difference, but whilst recovering from that trauma I'll keep them here too. ;-)
If you want to know what I am talking about, see the Notes in https://www.thingiverse.com/thing:3732450/files.

The original F623ZZ 3x10x4mm flanged ball bearings can be used with both verions.

Uses:

• with 3 wheels, like in NickRimmer's design. Then you'll have to drill the 2 upper ø3mm holes into ø5mm holes.
• with MGN9 rails + MGN9H sliders, like in my "3-wheeled Y guide goes linear", www.thingiverse.com/thing:2701748.

Edit (07-02-19): added Y-Carriage-2 and changed pictures
Edit (26-02-19): added .step files
Edit (21 aug 19): changed text above and actualized picture.