kaisa, gave me an excellent idea, easy and cheap.
You get some plastic mesh from stores like Michaels. They have variety of colours, so that is nice
They you get some "slide grip report covers", Staples and some dollarstores have them. You cut them and the mesh and just slide the mesh inside. Takes 5 min and cost a bit over a dollar I am very happy (thanks a lot, kaisa!)
My friend bettagirl did this as well she used silicone to seal the slide grip report covers to the tank it would have to be silicone that is aquarium safe I am sure some of the local fish stores carry it
You can use silicone if you like, but if you are like me, that doesn't want it permenent, it stays like that pretty well. I guess if you cut it right there is no way to move it
I have mine for 10 days now, works like charm Especially if you have gravel on the bottom. I guess you can use cups too. But I personally don't think you need anything else than the slide covers
But it is really easy to be done, takes a couple of minutes Or a few more if you wanna be very precise with cutting And VERY cheap. 1 sheet of plastic mesh is $0,89 and 6 slider are $2,6, makes 3 dividers if you use just 2 as I did (missed the bottom one as my tank has gravel).
I know this thread is old but wanted to give it props. I'm getting into bettas and used this idea for my 10G. I'm not permanently securing the dividers to the tank so keeping my "cheaper" bettas in them in the rare event one betta gets through the divider. It looks pretty secured and I've used small clay pots at the base to help keep the dividers in the place. I will be keeping my bettas that I am getting from Thailand (via transhipper) in my 5-8 gallon tanks (one betta per tank). Pics eventually
I'm using black plastic mesh (same type as original pictures but different colour). I find it helps with the stress but still allows heat, etc. to circulate the tank. Hopefully, the OP of that thread will repost too!