In which case you need your head examining.
2.5 gallons is not only a
far too small amount to keep a betta in permanently, you have a 5.5 gallon tank which IS the correct minimum amount. Why on earth would you try to cram a betta into an
unsuitable amount when you have a decent suitable tank which is perfect for a bettas minimum needs?
2.5 gals will be more of a faff to look after since a decent flter and heater to fit it will take up a good bit of the already too small space, and the water parameters will be even more difficult to keep stable because you have a lot less water to work with. Even with a filter you'd be needing to do large water changes every 3 days or so . You'd be making a lot more work for yourself and keeping a fish in what is effectively a puddle in a box.
A betta is a fish like any other, not a desk ornament. Would you keep a dwarf or honey gourami in 2.5 gallons?