Thanks for all the great answers! I appreciate all the advice and warnings. I'm well aware of the dangers, and quarantining, and all of that, so no issues there.
The 5.5 gallon tank I bought (I wish it was bigger, but I don't have space for anything bigger, and really wanted to rescue this betta
) comes with a black "hood" that's basically a lid...so it lays flat. I plan on making a divider out of acrylic that will easily go all the way to the top, and my husband believes he has a way to safely make it opaque (basically it's a type of sticker without adhesive that just attaches with water). I KNOW that if my EE boy can see another betta in his tank--at all--he'll go wild trying to get at it, or hurt himself if I use craft mesh. Bad enough that I had to put black paper on the side of the neighboring tank so he'd stop tail biting (and hey, it worked).