Look for a plant that has leaves wide enough for the betta to rest on. Anubias Nana has been doing very well in my Eclipse 6. It had maybe 6 leaves when I got it about 2 years ago, and last spring it was so big I took it out, cut it in half, and put one half back in the same place and the other across from it. It was a wee bit shocked at first -- but both halves are doing well again, with new leaves and new roots. I also have a crypt and a small temple plant, plus some anarchis. Inhabitants are, currently, three head & tail light tetras, each about 1" long. Oh, and small snails.
I got some white plastic craft netting in the craft section at WalMart, intent on making a cover for my 2.5G tank to keep Bob from jumping out (but now I don't think he's a jumper). At http://www.ultimatebettas.com
, a Betta forum, someone posted a picture of that kind of craft netting turned into a tube for their Betta. I made one immediately, about 5" long, and cut a hole in it (about 1"x1", about half way in) for Bob to have another access point. I made sure all edges of this cylinder were smooth. I hold the cylinder together using plastic bag ties, and I use ties to hold the cylinder in place just under the surface (though I'm pondering better ways to hold the cylinder).
Bob enjoys flowing through his cylinder, weaving in and out of it, and he does the same weaving through the leaves of the Java ferns attached to rocks on the bottom (mostly smooth rocks). He's a real explorer. At night he rests in the cylinder, and when he needs to breathe, the surface is right there, an inch or two away.
So feel free to give your betta things to play with and to rest on, because the Eclipse 6 is a rather deep environment for such a fish. As he gets used to it and the filter flow strengthens him up, he'll love his home.
Oh, and if you introduce other fish to the E6 before your betta, he may see them as having better territorial rights than he has and he may be less likely to fight with them.