Not really "DIY" but I figure you guys will know.

Please recommend a company if you have direct experience of using them. I'm looking for quality rather than cheap. Thanks.

call ralph from https://www.grafinger.co.th/en/home.php

 

 

From my experience, ask them do they guarantee that there will be no leakage from rain. Pay 50% at install and tell them you want to test it on the next big storm. then see if it leaks. Or hold a hose on the window are installing and the silicon is dry. Ive had new windows leaking in amazing thailand. Really annoying