so, ive been looking into a travel cage for my ferrets. lately on the fml there have been people posting about great travel cages, but unfortunately that info doesnt help me much. mainly because i have 5 ferrets instead of 2

plus 2 of them are big boys, and little piper is huge now too.
im trying to find a decent sized travel cage for my guys, that would comfortably fit all of them, plus still fit in the back seat of my car. preferably it would be one that folds down for storage, but i'll make due if it doesnt if the cage is amazing. the cage would be for overnight to a couple of nights trips, and i would bring along the playpen for non-ferret proofed areas. i know some of you might suggest to just use the playpen, but i have an escape artist in my midsts, and if im not right there, she's up and over no problem.
so far, heres what ive been looking at:
dog crate- the metal kind (not plastic carriers). i like the idea of them cuz most can fold down flat, they have big doors, and they come in various sizes. the problem? im not sure what size to get. i can outfit a larger one with levels, bedding, etc. i just dont want to get one thats either too big or too small, as they can be quite pricey. i'll most likely get one off of craigslist, but if i dont have a size in mind im not sure wht to look for. again i want to give them some room, but not go crazy with a great dane sized crate
martins cages - ive only heard great things about these cages. but again im still just not sure what size/style would work best. some of their larger cages do not fold up, so that would be a problem. the only other problem might be the price, but it may be worth it in the end if it means they will be more comfortable in it
any other suggestions would be appreciated

i know some people use the superpet cages, but i just dont like them. i had a big one originally before my ferret nation and i hated it

so id rather stay away from that.