I bought the largest cat/pet carrier I could without it being a dog crate, but I have two large lops to lug around.
I can't remember what make it it, but I'll look tonight. It has an opening at the top and the front, which is always really useful at the vets. Both of my buns hide in the corner of the box, so it would be really had to get them out if I hadn't got a top hatch to reach in a pick them up. The whole of the top half is also detachable, which is great for cleaning up little accidents and keeping it hygenic.
There are loads of different ones on Ebay, so have a brows around.
I don't like the soft-sided carriers around at the moment, they feel really flimsy I don't know if anyone else has success with them, so it's just my opinion. Best bet would to invest in something that will last ages and cope with living in the garage or loft between uses.