This is a 3-gallon round tank inside our 20-gallon aquarium. It has a ceramic soup mug under it to lift it up high enough that the surface is level with the water surface in the bigger tank. At first I had it sitting on the bottom but (thankfully) remembered that beta's need to surface regularly.
The beta has plenty of room to swim around. He gets to interact with the guppies and tetras without any harm. The guppies will sometimes gather and watch the beta like they're at Sea World or something.
Betas don't usually feel threatened by smaller fish but the guppies seem to be big enough that betas will see them as a threat and attack. I tried putting a beta in with an oscar one time, thinking no way will the beta go after that big ole fish. Wrong. That poor oscar was just minding his own business and the beta went after him like a Tasmanian devil.
I put plastic canvas craft mesh on top of the beta tank to keep him from going Evil Knievel over the side into the main tank. They will if they get the chance. I melted four holes into the top of the plastic tank with a soldering iron so I could secure the mesh onto the tank with twist ties. There is a small hole for the air stone tube (air set low so the surface is calm enough for the beta to make his bubble nests). The pellet food just drops through the mesh.
This is the plastic canvas I mean. Like for yarn crafts, etc.