I would definitely recommend soft carriers over the hard plastic ones. I find piggies are much calmer with the soft surroundings, they are more comfortable to carry and you can sometimes sneak the animals into shops and pubs and such without anyone noticing!
The first type I got was a Sherpa Bag similar to this one
http://www.discountedpetproducts.net/ORIGINAL_BAG_BLACK__MEDIUM__17_X_11_X_10_5-UKP60184.html. They're not cheap, but are very sturdy and easy to clean. I used mine regularly for about 5 years, but made the mistake of leaving it in the garage for a bit and the zipper got rusty. Another advantage of Sherpas is they have safety straps so you can attach them securely to car seat belts.
I've also got a T Bag back pack like this one, which is okay, but not as spacious as I'd like for two boars.
http://www.canineconcepts.co.uk/cgi...g=prod&ref=rucksack&cat=&catstr=&reviews=view I haven't worn it as a back pack though, only carried it. It's a cheaper alternative to the Sherpa. They also do one that looks similar to the Sherpa but I haven't tried that one. [ame]http://www.amazon.co.uk/Trixie-Tbag-Pet-Carrier-Small/dp/B000VBPEEK[/ame]
I got a lovely bag last time I was in America at K Mart, which folds up very small and also has the seat belt straps, but it's quite thin and not terribly sturdy, so wouldn't be that good for public transport or a long walk.
I recently bought this, which is gorgeous
http://www.zooplus.co.uk/shop/cats/...travel/fashionable_cat_bags_and_purses/217618 but sadly not that practical. Having small holes rather than mesh means the hay tends to spill out the back.
Whatever bag I use, I tend to use either layers of newspaper wrapped in a towel or even better, a puppy pad covered in a towel, at the bottom. Occasionally there can be a bit of "leakage," but it's easy enough to clean up with disinfectant spray, damp cloth and a bit of airing out.
Best of luck in your search!