I know you already got mesh, but large sheets of plexiglass are cheap and easy to work with. I'd guess a piece large enough to divide a 75 gallon aquarium could be had for less than $10. You need a hot wire or something to get a really nice cut, but any hardware store should be willing to cut it to size for you for less than a dollar; otherwise, you can get decent results with a standard saw and a little time with sand paper to remove the melted plastic burrs.
You could drill some holes in it for filtration purposes, or just run two smaller filters, one on each side.
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