What backing for matted prints

[Analog6]

to sell at the local markets

I have a fair range of precut mounts I have bought and am printing shots to go in them, and plan to take these and some calendars to a local market and dip my toes in the water. But they do not have backing sheets, they are just the edges with a square/rectangular hole. What weight/type board should I use?

[ricktas]

I use backing foam. get it from the same place as my mats. I have a large range of sizes in mats, all pre-cut, with the backing foam in place to fit, then they are stuck together along one edge using acid free double sided tape, to create a hinge. So as soon as I need one, I grab it, open it up, place the photo in, and then use more of the double sided tape on the remaining three edges and stick down. Easy