Hi again,
Trend Router Bases are tapped M6, so you only need a clearance hole and counterbore.
Plastic/Nylon/Wood would also be ok to use. The Trend one is probably plastic, so it won't mark the job.
For Reference the piece in this picture is 70mm long x 20mm wide.
The Guide Bushes are flush with the router base, so don't add anything to the thickness of the jig.

Regards,
Mark