To be honest, DIY are about the same price as the pre-fab ones, once you get the silicone to apply them :/ And theyre a bit flimsier...
I recommend doing what I did. I bought a fre-fab kit from Top Fin (PennPlax makes good ones too, sold at petsmart/co, respectively) Theyve got ones for 10 gallon tanks, so I didnt have to mess witht he size of the spines (height-wise). They have little clip things at go under the lid, which is nice and secure- no glue needed.
The inside is clear, though, so I replaced it with that crafting mesh. Use a dark color like black (use two) to make it hard to see. Leave extra at the top (taller than the divider spines) so they cant jump; make it as tall as you can, up to the top of the tank's hood.
Put plants around the divider, if you want. At first, there might be initial flaring, but they get used to eachother...
Btw- bettas can see pretty well even from a foot away. That couple inches might do less than you think lol!