My mailbox is similar too this

Bert had a nice way, where he just cut a piece out of his mailbox with a sort of custom "flag", but his mailbox is from plastic and opens on the top.
Basicly what i want is a way to see that when i come home that there is actually mail. Ofcourse i can do this by turning on some light somewhere but i would
assume there must be a easier way to "detect" something like this. I could also send myself a message which only i get so that still leaves the WAF factor unsatisfied.
Any ideas to tackel this? I tried searching on "vlaggetje mailbox" on google, but all i got is finished american mailboxes, nothing that can fit on this.
Thanks,
p.s. so a modification to this mailbox to see physicly that i have mail without having to open it up first
