There are commercial dividers out there but I have never used one, so I can't express any valid opinions on them. Is this a temporary thing, or longer term? And is the look of the results important? Plexi glass could be an option, but you need to get some holes drilled in it to allow circulation. Some plexis drill better than others. Why are you trying to divide the tank, and what kind of fish do you have? Sizes?
My first thought runs to what some people call "egg crate" which is a completely confusing term because it has nothing to do with eggs, or crates. It is the white plastic grid panels sold in lighting stores as a diffuser for ceiling lighting. It generally has openings of about 1/2 inch square, and lots of them, so circulation would be great. It can be painted with Krylon Fusion to tone down the whiteness. Just let the paint dry for at least a week before putting it in the water. The egg crate can be cut with heavy scissors, but be prepared to sand off any sharp edges to protect your fish.