Some info from the old boy, I'll follow up my end.
The New Carcoon I use Blue & clear, I as you know have the 240 Sam in it, the car fits fine the Carcoon however wouldn't need to be any smaller, the car fits perfectly. Model is a size 4, 15'5 x 5'11 clear & Blue.
The original one we used to sell at Deutschetune was somewhat larger, as you know not only did your V6 fit in it but also my spare set of Z wheels @ 225/60/15! Works dunlops at that!
I bought this one deliberately to put the e'type in after I rebuilt it prob a size 5, 16'7 x 6'6 maybe bigger.
The new covers are much better than the original being a softer fabric, you may recall you need to pull the cover over the car to zip it up, always been an issue with the old cover when the clear cover became a bit brittle with age & the possibility of scratching the roof. The new one does not have this issue.
Another thing to consider, one of the frustrations with this cover is that when the car is in, it is there till you take it away completely, ie you can't just unzip it & take it out for a run without quite a lot of hassle.
I was looking at the bubble Colin Coats keeps his toys in (B Bently & the pagoda merc) this bubble is free standing & has a zipped front/ rear which allows you to simply unzip drive out & likewise drive back in without the hassle of disassembly. Carcoon now do one of these, all on the web site, it's called an indoor Veloce Carcoon. Actually quite fancy one myself, self explanatory! If any concerned re the overall effectiveness my Z speaks for herself, most of these come out of garages etc on the end of a brush (rotten) mine as you know is a 72 car, as was converted by Spike Anderson to Samuri spec in about 75/6 & has never been welded or painted.
If you drive the car in wet, the airflow dries it off so no real panic there.
I do recommend this (good sales pitch!)
You also asked about the bottom garage, really grotty this but functional being concrete however I made the mistake of going for the flat roof version, not good it (concrete roof) drips condensation. Now fixed with a new roof but would always go with a small pitch for airflow if nothing else. It is 19'7 long x 16'7 wide ( old money!!) the door is 14'x7' ,being mindfull that both carcoons fit in here with cars & albeit tight you can still walk between the cars.
It was originally a Marley garage, they have gone tits up but there are loads of cheap garages out there which will do the job fine, especially if the car has a Carcoon type facility.
Likewise I have never used a dehumidifier, none of the cartoons have ever misted up so never felt the need.
Finally the charging system that comes with the Carcoon kit is excellent, I have only replaced one battery in the Z since 1991, that's got to be good.
Sorry for the epistle but hopefully helpfull.