There are at least two different lids for the alluminum powerbook 15", based on the resolution of the LCD. The later, higher resolution models are a bit larger than the earlier ones. If the resolution of the model lid matches that of your book it should fit.

The logo itself is slightly thicker than the lid and slightly larger than the opening it sits in. there is a beveled edge that allows it to sit flush in the opening and leaves a bit around the edges on the inside to hold it in place.

As for the material I can attest that lexan or polycarbonate is much softer and easier to work than plexi/acrylic. Both can be gotten at the local building supply store.

Where I live in NYC there is a shop that custom laser cuts and engraves plastics, They are dirt cheap and super fast, usually cutting a piece while you wait. The can cut and engrave any shape or design you can provide. They do have a web site and you could contact them about possibly using their services by mail.