No supports needed :) Variable layer height produces a more comfortable handle. PETG is fine. Handles in the pictures were printed with default 0.2mm PETG profile and variable layer height. You may wish to print more walls to make it more sturdy so the file wont flex in the handle.

Assembly instructions (see the pics):

1. Take a printed handle and a suitable file
2. Hold the file with a leather glove or similar to avoid burning yourself, and heat the file's handle end, for example over a lighter
3. Try to fit the file to the handle. If you feel it would go in, push it until it bottoms. If not, heat some more and repeat