Further to what I wrote above, I asked my friend for details about his home-made car storage systems, and this morning he sent the following:
I have made two dehumidified bubbles, the first, in my work shop, consists of a very thin polythene body shop car cover attached to a wooden rectangular frame (just a bit bigger than the car) that sits on the floor with a rubber seal. The car sits on a rubber mat. I inflate the bubble with a dehumidifier that attaches to one end with Velcro. I can raise the bubble/framework off the car with an elaborate pulley system in the roof with an electric motor.
The second bubble I made at my girlfriend's house (picture attached) was supposed to be a temporary bubble that consists of a sheet of thick polythene for the car to sit on then a very thin polythene bodyshop car cover thrown over the car. To seal it to the ground I came up with the idea of "sand snakes" I sewed up some 8' lengths of vinyl and then filled them with sand. These snakes sit on the cover and seal it to the ground. Two down either side and one front and rear. I then attached the dehumidifier to the bubble with duct tape (no need to detach it) uncovering the car to drive it out only takes about two minutes. I can leave the front sand snake and dehumidifier in place and just move the others out of the way.
This temporary bubble has now been there for 18 months and is still working perfectly! I can take the car off the road for the winter and leave it there for 5 months without the disc brakes rusting at all.
My first bubble (the one in my workshop) has an air re circulation system that vents the air out of one end of the bubble and ducts it back into the dehumidifier. The bubble averages 45% relative humudity (anything below 55% is perfect), the rest of the workshop varies from 70% to 90%! This is no longer the case as I now dehumidify the rest of my 110 year old workshop.
The best car storage system in my opinion is the air chamber with its tent like internal framework. Unzip the front door and drive out. I intend to buy or make one of these at some point.
Airchamber Indoor Vehicle Storage | Indoor Vehicle Protection
It's very important that a dehumidifier is used instead of the fan supplied. I have not yet tested which is most effective, having the dehumidifier inside the airchamber with the fan hole sealed up or having the dehumidifier outside the chamber blowing the air through like the original fan. The air vents through all the zippers.
I have, however, tested my inflatable bubbles with just a fan and found the humidity inside the bubble to be the same as the rest of the workshop; the only benefit is the circulating air. This is a big benefit over still, stagnant air but not nearly as good as dehumidified.
The car storage system I don't recommend is the Carcoon
Carcoon Storage Systems International LTD - Carcoon Home Page
This is because it takes a long time to get the car out and the plastic is very thick and heavy to roll over your paint every time you want to drive your car.
I have spoken to the people that sell both of the above products and they all say the same thing; you don't need a dehumidifier...BS! They also say it will make it too dry, insisting it will dry out your leather, rubber seals etc..BS! I have kept my Pantera in my dehumidified bubble for 10 years at 45% relative humidity and have suffered no ill affects at all. I can't understand why I am having to replace my tyres every 6 months though.. Just kidding!
Okay, so there's some first-person testimony from a guy who's been there, done that on the cheap. His comments regarding the potential for damage to the paint are worth noting--he's well-known in the UK for his fanaticism regarding the care of paint and bodywork (he's the Euro importer for a high-end car wax called Harly Wax).
I don't have to contend with these problems myself, since I live in inland California, where it's hot and dry much of the year.