Antenna for 868 MHz. I use it for a project: https://meshtastic.org/ .

Antenna calculations in one of the photos, in the same place the result of checking the parameters. For assembly, a brass tube with a diameter of 4 mm is used.

If PETG printing is not possible, choose other refractory plastics. If you are limited to choosing PLA, print in white so that sunlight does not heat up the structure so much.

You can check plastic for radio transparency using a microwave oven. Place a small piece in the oven and try to reheat for 2 minutes. If it gets hot - do not use it to print this design!