If the keezer you would DIY is in the basement or garage and doesn't need to be a show piece, then you can cobble together a 9" tall collar from a sheet of cheap 3/4" plywood and 4 9" pieces of 2x4's for the inside corner pieces. you don't need to glue it. just get a couple of clamps to hold the first corner together then screw it together with 1 1/2" drywall screws. then move the clamps to the next corner and repeat. it doesn't need to be pretty, and you can fill any gaps due to slightly imprecise cutting with caulk. in case you're wondering, the main reason for the collar is so that you don't risk drilling through the cooling coils when you go to install the shanks for the taps. the side benefit is that you can fit a fourth keg on top of the compressor hump.
I have built 2 of these (for 7 cu ft freezers) and I've used 2" thick styrofoam on the inside of the collar for insulation. but before you install the insulation, install your taps and shanks so you can really tighten the inside nut on the shank so the tap doesn't move. you can't tighten it against the styrofoam. the first one I built I glued the styrofoam in place, but that made it a pain when I needed to remove it to retighten the nut on one of the shanks. just setting the styrofoam in place then give it a small, removable. dab of caulk on the inside corners will be all you need to hold it in place.
it will be a little tricky (depending on the brand of freezer you buy) to remove the lid of the freezer. they don't put them together so as to be taken apart. but with patience you will figure it out. then you will reattach the hinges to the collar. you can use FreezeKing brand foam weatherstripping to place between the top of the freezer sides and the bottom of the collar, and between the top of the collar and the lid, so you don't have any air intrusion due to an imprecise cut on your plywood.
I use an inexpensive Johnson Controls thermostat. you plug it into the wall then plug the freezer into the thermostat's plug. turn the freezer to max cold and it will be sure to turn on when the thermostat tells it to. the capillary from the temp bulb to the thermostat is tiny so no problem closing the lid on it when the bulb is dangling inside the freezer.
I drilled a hole on the back of the collar to run the CO2 hose through so I don't need to have the CO2 tank inside the keezer. no need for that.
final piece of advice from experience is to buy a 3-step ladder. a full keg is pretty heavy to lift over the top edge of the collar and gently lower into the keezer. I first set the keg on the top step, then step up onto the second step, lift the keg onto the top of the collar, step onto the top step and then gently lower the keg. when I'm 80 I will have to consider installing a powered winch attached to the ceiling to do the job!!
DIY will make sense for you if you really can get all your kegs for $100. if you have to buy them, even used, 4 will cost at least $300. but still not a terrible expenditure, since 7 cu ft freezer prices have returned to earth since everyone that wanted one last year has bought one.
