Just make sure it isn't made of shiny material or else you can get hot-spots from the lights glaring. I agree that something like the Photoflex FlexDrop2 would do just fine.
They're not collapsible frame green screens like the Flex Drop (Photoflex) but these guys build a very sturdy, light foam backed material that is mostly non-reflective and you can get it in larger than 5' x 7' sizes.
Depending on either scenario, you'll most likely need some sort of frame also to hold the screen so either Photoflex's or EEFX's materials will work well.
I have two green screens from eefx.com and they are superior to anything I have ever used. You'll love 'em! I only use them in the studio but they're great. I still think a collapsible is best for quick setup on the road.
I just made one a while back. One of those spring loaded roll-up type windows blinds spray painted flat green, hanging off of one of those IV bottle hangers on wheels that you find in any hospital.
One low tech item I've found to be very effective as a portable green screen is using flourescent lime green lycra (Spandex). I have a 4'x7' piece that I got on sale at a fabric store. It requires very little lighting, lights evenly with very little spill and wrinkles stretch right out. It can be taped in place. For fast set ups it is my screen of choice. It can be found on ebay for cheap. I have a fold out portable screen and a 4'x4' piece of the velcro type material but I find myself using the Spandex the most.