They make both indoor and outdoor versions, the outdoor version being aluminium coated double skinned and offers better UV protection. They are permanently hooked up to the mains in order to power a small fan that keeps the filtered air circulating which prevents condensation forming and so preventing rust (you keep the windows wound down to allow air to circulate inside). They estimate that it only costs around £10 a year to run plus filters. It has other advantages such as protecting it from cats and birds (and idiots who enjoy scratching cars), the UV protection prevents your paint from fading and it keeps dust and other airborne contaminants out. An added bonus is that it acts as a battery conditioner. So all in all seems like a good investment.
You obviously need a drive to put it on, don't think you could put them on the road even if you added reflectors but could be wrong. The downside, if you exclude the fact you can't do work inside them, is that they take a bit of time to get the car in and out and ideally it's a 2 man job. So it wouldn't work for a daily driver but that's not a problem in our case.
If you want to be doubly sure your car isn't damp then you could also buy yourself some silica desiccant, the same stuff as in those little bags that come with electronic goods, which will absorb moisture and a thermo-hydrometer to measure the humidity, you want to try and keep it below 50%. I'm guessing you'd want 2.5KG (2500g) of silica which absorbs around 60% of its weight in water, so about 1.5 lires of water. A smaller quantity would be fine it just wouldn't last as long. Neither are expensive and should only cost £35-£40 from eBay. The good thing about silica is that it can be reused over and over, just place in a warm oven for a few hours every few months (or when the humidity stays above 50%) to dry it out.