I've divided quite a few tanks what with betta males and other fish.
Used both plastic canvas which you can get from the hobby shop and also perspex which I got from a local hardware shop... cheaper than B&Q as well.
I used sliders for the perspex ... comes in single strips of around 7foot and you just cut off what you need and glue it into the tank, cut the perspex to the size you want and drill some small holes in it to allow water flow but too small for the fish or larger shrimps to get through and there you have it a divided tank which you can by simply sliding out the perspex make it a large tank again, dead easy.
As for plastic canvas if the piece you have isn't big enough for the tank you can stitch it together using plastic scooby laces, get the document spines from a stationery shop and glue them into the side of the tank and then you can slide the plastic canvas into the spines (better fit than using sliders), once again these can be removed as and when you want to change the tank arrangement.