You planning on keeping those rasboros?
The way you're set-up now, the water in your cycling tank is probably as good or better than in your 2.5g.
If I were doing it, I'd get a few live plants, floaters like water sprite or hornwort to eat ammonia. Get a couple of good hides. If the Rasboros are going to be tankmates (?) then introduce your Betta,
Check parameters daily and change out any ammonia or nitrite over 0.25ppm. It'll take a while, but it will cycle eventually; it won't be any more trouble than what you're doing now---probably less---and arguably better for your fish.
If you're experiencing noticeable evaporation, your tank is open. Betta like warm moist air on their labyrinth organ. Even a simple cling-film cover is nice for them.