Very strange, because I keep a Betta in a 40 litre tank, and I have also been looking at setting up a new 'Shallow tank', but I have decided I will probably need to use a larger tank and only half-fill it to achieve what I want to do, as I haven't been able to find a shallow tank with a lid, to prevent the Betta from jumping out of the tank, and you also have to consider how you will have your lighting set up.
I would be interested to know which tank you go with, I am looking at the Tetra Starter Line 110L, as it is 78cm long (6.5 inches longer than the 2ft you describe). I would have it filled up with around 60 litres and deep substrate and still have room for taller semi-submerged ornaments, and plants to grow.
The only downside is the lid is not transparent.
As for filters, with the size of tank you are describing, and if you're not overstocking (you wont want too much in a tank that size with the Betta) I think your best bet would be a small sponge filter.
Let me know how it goes and what tank you go for, as I have been looking at the exact same thing and will start my own thread soon.