Plexiglass would work, or you could buy wood and cut to size. I would go with ubecrew610's advice and use auarium sealant. Regular caulk would PROBABLY be ok, but the auarium sealant will deffinately be non-toxic.

You could also buy trim, cut it to the height of the tank. Cut four pieces and affix them to the tank with enough space to leave a groove between the two pieces on either side. Then you would have a removable divider.

If you have trouble with humidity or ambient temps let me know. I found a great solution that is relatively inexpensive. I was able to fix this problem on all my aquariums of various sizes for a total of about 30 dollars.

Good luck!