My experience with betta bowls has not been good, mainly because you need to do a daily water change to keep it clean enough for him. Also, it tends to get that weird scum on the top, which is part house dust and part bacteria, I think.
For a 5 gallon tank, I wouldn’t worry about any kind of filtration, as long as it was just the betta. Just do a 10% water change every week, by scooping out some old water and putting in some new water. In the wild, bettas live in still water, so he doesn’t really need a bubbler. When you see the bottom has gunk all over it, scoop him out with most of his old water (from the top of the tank), put him in another tank or bowl, and scrub out the tank. Cover his temporary home so he can’t flip himself out.
And watch the pH if you live in an area with really alkaline or acid water. It can creep up (or down) on you. I don’t recall that my bettas were really picky, but once it got to a certain level, they just weren’t happy. Our water here (downstate Illinois) is about pH 9.0 straight out of the tap. All the reputable fish stores in the area have signs warning people about this. You add sodium biphosphate to lower the pH. I’m not sure what you do if it’s too acid to begin with.
Another concern with a betta in a tiny tank is temperature. He wants it warm. If your office is nice and warm, then you won’t need a heater. But if it isn’t, then go with at least a 5 gallon tank and the smallest size heater. It’s difficult to get a really small tank like a 2 gallon to stabilize with a heater. There’s not enough water for fluctuations, and it’s always either too hot or too cold. The thermostat needs time to expand and contract and send the message to the heating element.
Also, watch it carefully for a week or so before you put him in there, because before the thermostat gets calibrated to your room conditions, you can look over there and realize that he’s basically cooking in 85 degree water. Give him his very own thermometer (and don’t waste your money on the heat-sensitive self-adhesive ones–they really suck. Get a regular hang-over-the-side metal/glass-tube- with-alcohol thermometer, they’re more reliable.)