Reddit reviews Bulk Buy: Darice DIY Crafts #7 Mesh Plastic Canvas Black 10.5 x 13.5 (12-Pack) 33900-20

That is a custom piece I added to fill the empty space up front :)

Bulk Buy: Darice DIY Crafts #7 Mesh Plastic Canvas Black 10.5 x 13.5 (12-Pack) 33900-20 https://www.amazon.com/dp/B00KHAYGJO/ref=cm_sw_r_cp_api_i_OXCsDbK79MCF8

Not OP, but I use something similar to this:
https://www.amazon.com/gp/aw/d/B00KHAYGJO/ref=mp_s_a_1_1?qid=1466054396&sr=8-1&pi=SY200_QL40&keywords=black+plastic+mesh&dpPl=1&dpID=610lq-Q-uUL&ref=plSrch

Can be found in Walmart in the sewing/crafts isle for less than a dollar a screen.

Bulk Buy: Darice DIY Crafts #7 Mesh Plastic Canvas Black 10.5 x 13.5 (12-Pack) 33900-20 https://www.amazon.com/dp/B00KHAYGJO/ref=cm_sw_r_cp_apa_i_xXZwCbPKT2TAQ

13$ and you'll be set for life.

DIY. I used the following to make it:

"Hinges": https://www.amazon.com/C-Line-Report-Covers-Binding-32557/dp/B000GOZZI2/ref=sr_1_fkmr1_1?ie=UTF8&qid=1495438160&sr=8-1-fkmr1&keywords=report%2Bbinders%2Bblue&th=1

Dividers: https://www.amazon.com/Bulk-Buy-Plastic-12-Pack-33900-20/dp/B00KHAYGJO/ref=sr_1_2?ie=UTF8&qid=1495437902&sr=8-2&keywords=plastic%2Bcanvas%2Bblack&th=1

Sealant: https://www.amazon.com/Aqueon-Silicone-Aquarium-Sealant-3-Ounce/dp/B0002ASD2U/ref=sr_1_2?ie=UTF8&qid=1495438401&sr=8-2&keywords=aqueon+aquarium+silicone

Glued the hinges to the glass with the silicone (any gaps leave on the bottom since sand will cover that), let it cure for 48 hours, then added sand, then cut the dividers into shape (11.75x10.5), and inserted it into the hinges. I have pretty deep sand so it's not an issue, if you need a larger pieces you can probably find them somewhere else or stich smaller pieces together with a nylon thread.

Overall it holds pretty well, gives water flow, and after planting the future bettas should only be only vaguely aware of their neighbors.

I don't recommend buying divided tanks for a few reason, 9/10 are too small, the remaining 1/10th will cost an arm and a leg for something you can make for 30 minutes of work and generally have poor ratings (take the infamous divided tank sold by deep blue). Final issue being that the dividers are generally glass or plexiglass, anyways - not opaque.

The sealant is a small tube but it its enough for the job and any touch ups. Comes with a twist cap unlike the calk gun containers meaning that you can close it and reuse it later when need arises.

Plastic canvas:
https://www.amazon.com/gp/aw/d/B00KHAYGJO/ref=yo_ii_img?ie=UTF8&psc=1

Report things:
https://www.amazon.com/gp/aw/d/B01COD03UM/ref=yo_ii_img?ie=UTF8&psc=1

I'm sure you could get them elsewhere cheaper, but my walmarts, targets, kmarts, etc and craft stores didn't have them, at all! So I just ordered them with prime.

Also, zip ties and plastic mesh. The less debris that gets inside the better.

Here is the desktop version of your link