what if I used, for instance, a 2 liter soda bottle instead of that
huge 5 gal jug? and does all the co2 really make THAT huge a difference?
as in, would it be anywhere near as successful if I skip that part of
the setup?

You could use a 2 liter sure. I've used those 3 liter bottles for the co2.
Does it make a big difference? Honestly I don't know, but the cuttings will be in an enclosed space- Don't know how much they breathe as they do they'll use up teh co2 in that space. If it's not replaced (open the lid, make some, etc) they'll die.

what use exactly was the cap on the jug, other than airtightness?
would a hole drilled in a bottle cap and securely duct taped be sufficient?

It's just airtight. If you use fish-airline hose just drill the hole a little smaller than the hose & force it through. It'll seal well enough.

would 2 23 watt CFLs be sufficient to light a box maybe half to a third
the size of yours there?

Sure- At least I think so. I use 2 of those bulbs above each of my seedling flats & the kids do fine.

would it be enough to line the inside of the lid with the shiny side of
aluminum foil instead?

Yup. Mylar is just alumanized plastic. Get it near naked electricity & you'll find that out really fast.

what's the significance of the hydric acid vs water?

Hydrogen Hydroxide, Hydronium Hydroxide, or simply Hydric acid. Its basis is the unstable radical Hydroxide, the components of which are found in a number of caustic, explosive and poisonous compounds such as Sulfuric Acid, Nitroglycerine and Ethyl Alcohol....
It's a chemical name for water. Just plain water. H2O.

I can't really tell from the picture - how airtight is the lid, if at
all? and how did you make it?

It's the lid that came with the box... So I don't really know how airtight it is. Somewhat to not very, I'd guess. Like it might keep out ants but not keep in mustard gas.
If the co2 is going in the pressure must be equalized- An equal amount of air must be able to escape or you have a popping risk. (I'd say "detonation" but.. betcha that co2 can't attain much more than 10 pounds of pressure.. Which might make something pop rather than detonate.)

thank you so much for this post. I'm gonna build it tomorrow.

so if I entirely omit the co2 production chamber, and use a box with small vents, it would be fine? should I mist them every now and then? or like, I don't know, BREATHE into the vents? LOL.

it's also pretty cold inside here, like, 60 degrees if we're lucky. think they'll still root?

thanks so much again!!

Ria

Ya you can toss the co2 setup. Make some holes in the top- 1" or so.. Larger if you don't have small critters (ferrets) who might like to climb in there & "help". Just put the lights outside the box so it doesn't get too hot inside.
They'll help heat the box anyway.. So I wouldn't worry too much about what the temp is inside. Too hot = bad, too cold.. Not so much.
I have a little temp / humidity station I put in mine just to check.

Welcome-

I do have a small kitty, but she probably won't bother anything.

how long should the lights be on/off?

thanks so much for all the advice :D I'm gonna try building this pretty soon.

Ria