Here’s some pictures especially for carl3- seeing your website was funny- our mouse cages and racks are so similar! I just took these this morning and then I saw your post here, and it was like, didn’t I take some that looked like that today? I like your idea about the sign making stuff for shelves- you swear it doesn’t warp eh? I might have to try it myself.
.
Simple roll-around rack with sweaterbox size. I use 3/8 smooth plywood for the shelves, paint the cut edges so they don’t swell up and ruin the fit. I do put a strip above and below the shelf to help keep it from warping and pegboard on three sides so it can’t tilt or wobble even if empty. It is pretty solid. I use a piece of “doorskin” for a spacer when assembling the next shelf, the fit is not so snug that the plastic is scraped every time it slides in and out. Well maybe a little...

.
Rack details- I don’t get lidless systems, You have to periodically scrub the “ceilings” of each shelf? Maybe it is easy to pull the cage half way out to clean a poop or something? This system with lids is virtually escape proof, and in my case, very important...idiot proof!

.
El Cheapo .99 cent shoebox mouse cage. I just bend the edge for a lid, “sew” a hopper with wire, use two wire “straps” to hold the lid down. Total cost with critter sipper is probably about $4.50. I use these for one or two mommas to have litters in, or several real young that grow here until getting fed off (males) or become breeders (females).

.
Another view of a momma’s house. Takes about a half hour to make one of these. This one has a strip of ¼” mesh around the top edge about four squares wide so they can’t chew the edge. I don’t bother with this anymore, I would just replace the .99 cent tub every now and then.

.
Bigger shoebox for one male and three females just for breeding. These females get put into individual shoebox cages when they look prego or after a week, or as I rotate another batch of females. Lucky guy...

.
Close up of my wire feed hopper. There is a strip of larger mesh at the bottom of the “v” so mice don’t chew through the mesh to get food. Works like a real food hopper should. Simple mesh and sewn together with small gage wire I bought at RagShop for under $2.00

.
Male and three females. I use a layer of kitty litter, whatever brand comes in the plastic jugs, then some aspen. I flavor the water with vanilla because I heard it helps the smell? I really don’t have much odor from them at all unless I have a dead one over the weekend and I haven’t checked on them- it happens.

.
I am planning to build a breeder rack. It will be unique in that there will be two sweater boxes per shelf, with lids. There will be a matching hole in the lid and floor so the two are really one cage with upper and lower areas. Nothing connected permanently except they nest together into the rack space. You have to slide both out together, then you can get to the top part or lower part like two separate sweater boxes. Except the top one has a hole in the floor, and the bottom one has a matching hole in the lid.
Anybody have such a thing with sweater boxes?