Pen to write on acrylic paired surface?

[Miljea]

I've made three deep sided 5" square little paintings (naive tree with hearts!) for three friends. I want to personalise them with, say, 'Welcome Home!' So I'm looking for a marker pen I can write the message around the 1" deep side. The 'ink' needs to be dense/opaque, but the edges must be crisp so I can be a bit 'calligraphy', but a shaped 'nib' could be 4-5mm wide/thick.

Any ideas?

TIA.

Aaaand- I didn't get to proof-read the title 🙄.

I meant an acrylic painted surface!!