I will tackle that when it is warmer. I already sourced grommets for Mercedes that have a great design, water can flow immideatly outside with no need of accumulation. The need a hole of exactly 19mm diameter which I will perform with a crown drill. Corrosion protection of course.
It will secure the home of M534 against malicious events, of course, this does not obviate the need for regular drainage checks...
Btw, the cheapest way to source them is directly from a Mercedes dealer, the number is all you need