No, you can not use suitcases to transport ATM cassettes to Koh Tao. If you do so, you do not follow the approved Group procedures, and you are therefore NOT covered by insurance (a suitcase doesn't float, a Pelican box does).
If it's not possible to drive the AV on the island, you MUST use Pelican boxes. It's mandatory. As far as I remember, we have already advised you to re-arrange the number of Pelican boxes among the Branches, as some of them have more than they use. If it's still not enough, you need to anticipate the need and buy new ones.
I remind you that GPS are to be used too.