IF you are never going to use them above another box, They are simple. Mine are all top bee space so even if'n I want to they are OK, as far as bee space is concerned.
Plans? Just do it!
Arrive at 19mm thick timber (planer thicknesser or buy in PSE). 18 to 20mm is OK.
Saw to required width (planer thicknesser again, if you have one - but sawn is good enough).
Cut side walls to length 460mm (could cut longer and trim to length later, but just so easy to set up a stop for multiple ekes).
Route two 19mm (timber thickness size) grooves on the sides with 372mm between the grooves (inner measurement, not centre to centre, so 25mm (+ or - ) between end of side and outer edge of groove.
Cut ends to fit giving 460 outside dimensions when firmly seated in the grooves, (too long not good - cut a bit more off, too short not good - fill groove with a suitable filler strip, just right - spot on!).
Assemble on a flat surface, one screw straight through for each corner. When satisfied they are square (some assemble on a queen excluder) and flat, glue up, add another screw at each corner but 'on the tosh' and leave to cure. I usually use a thin screw in the centre as a 'locator' then screw in two screws on the tosh.
Make end rails to butt inside the side walls - these can be more light-weight (thinner section) than the end rails on the deep boxes you are converting (never really any load on them). Nail or screw rails from the inside (fixing on the tosh, again) after a single screw through the side into the rail. Preservative and surface coatings (I varnish mine).
Job done.
Fit to bottom of deep box, with screws upwards through the rails.
If you must arrange for the bottom bee space, assembly can be done onto/into the inverted deep box (slightly thicker bottom rails will be needed to be sure to cover the effectively narrower end wall) or saw the timber narrower by your 'bee space'. They are a doddle to knock up, preferably several at a time.
Just tend to make them a mm too wide than finish a mm or two under on the inside measurements, otherwise you will always catch that lip with frames.
I use screws as though they are going out of fashion - far more than I need. Most could be removed once the glue has cured and the eke is fitted to the box. I don't like nails - shades of a site chippy! If you only screw and don't glue, they are easy enough to take apart. I think I only dabbed some glue on the butt joints on the rails (keeps water out of joint.
Cut for some extra, 'cos you will be changing your other deeps to 14 x 12! Stored flat pack, the extra outlay on the timber is not a lot. While you are at it, cut for some other sizes (for apiguard or feeding per eg) Narrow ones can be simple butt joints but 75 mm ones can be used as pairs for a super, or a shallow plus an eke for a deep brood (OK won't need that as you will soon convertt hem all to 14 x 12).
I have considered using only simple butt joints throughout, but always succumb and make that slightly better job. None have failed yet in about 8-9 years (although the first ones had tenon joints on the rails).
Regards, RAB
