Don't know what your tooling requirements are but I've found this spring loaded engraving tool from F.E.T. is the Ultimate and Most Affordable "Floating Engraving Tool" kit available! works very well on irregular surfaces - particularly cast acrylic.
This gives me a nice depth of cut even if I have to go a bit deeper to make an etch on an additional pass for sections that are even lower.
Might be worth a look.