Summary

Why yet-yet another? Well, there's tons for the 30A version of this PSU (various clones: Supernight, Mean Well, and several noname), but very few for the (fanless) 20A version. The one I have is the 12V 20A Supernight but it should fit several others (check images).

This design is inspired by a couple of others in terms of features, but is done from scratch. OpenSCAD sources included.

Instructions

Bill of materials (links to what I used, but you can substitute):

Voltmeter
Ammeter (see note below)
Rocker switch
Large grommet
Small grommet
Grounded power cord
Zipties (to secure cables behind grommets)
Wire
Crimp-on lug connectors (if you want)

Caution: Linked ammeter is under-rated for this PSU in general (10A vs 20A); however in my application the load does not exceed about 8.5-9A. If your load is higher, you will want to substitute an ammeter with a different shunt (such as this one, although fitting the shunt might be nontrivial; alternatively, just edit the SCAD source to remove ammeter cutout).

Instructions:

Print back cover (and, optionally, arm and front cover). I used a brim and enclosure. Since the back cover was a pretty long print @.2mm, I still got some minor warping and very tiny layer gaps in corners, which I friction welded just to minimize chance they'll delaminate in the future.
(Optional) Install grommets.
Tap holes for meters, and install meters.
Wire rocker switch and thread wires through mounting hole, but do not clip it into the cutout yet if you will be using the retaining hook (back arm). Otherwise the arm will hit the switch and you won't be able to slide the cover on.
Solder and connect wires. Mostly common sense. I guess you have two options for the voltmeter: (i) in parallel to PSU+ and PSU- terminals, or (ii) in parallel to PSU+ and ammeter shunt input wire. Not sure if it makes any difference (shunt voltage drop should be tiny), but I went for the second.
(Optional) Install zipties on cables behind grommets, for strain relief.
Connect all wires to PSU terminals. At this point, everything should be ready so you can slide cover onto PSU. If you want to adjust the voltage trimpot and haven't done so already, now would be a good time.
(Optional) Install back arm retaining hook. Press-fit an M3 nut on the arm with your soldering iron, then hook the arm over the PCB standoff (it's dimensioned tightly, you need to slide-and-twist on, and you may need also to file arm slightly). Slide back cover partially onto unit. You may have to guide the back arm into place with a screwdriver through the rocker switch mounting hole. Once in place, screw back arm onto cover.
Clip rocker switch onto cover. This requires some care, as it's possible to delaminate the part if you apply excessive force. Insert one side first, then the other. If switch is too tight (also depends on printer calibration), file the hole edges slightly.
Secure cover with a screw on the side of the PSU. You can use just this screw, and skip the back arm if you find installation too finicky (also: installing is ok, but removing is quite tricky, with rocker switch clipped on -- so, if you plan to remove cover often, it's best to leave arm off, or edit source and move rocker cutout ~5mm to the right).
(Optional) Front cover just slides on (it's only purpose is to make the bottom even, you can skip if you want).

For a DC connector you can use whatever you wish (in the picture it's a couple of quick-connects I had lying around), or you can do just bare wire.

Yet-yet another power supply cover