I think you could do it over two summers (depending on where). If there were two blocks. You could have a group stage block and then maybe knockout or final group stage. 3 weeks to 1 month - a pure test cricket window.
I'd have 3 groups of four so you can run two matches at the same time. Group winners plus the best runner up qualify. You could rotate the 'host' nation for each group every time.
A lot of people seem desperate to cling to the idea you need to play series but I think one-off winner take all test matches would be quite exciting.
I used to think two divisions for test cricket would be sensible but I reckon that'd probably destroy it or make it as exclusive as test cricket is in the women's game.