Building dipole antennas is always some kind of improvisation. Center insulator is quite simple and may be made in short time with jigsaw and drill.
As I jumped in CNC world I need some simple tasks to learn how to use FreeCAD and other software. So, dipole center piece is ideal just for that. I’ve finished project and learned a lot in the process.
Finished projects is uploaded to Thingverse: https://www.thingiverse.com/thing:4149187
You may use CNC mill or 3D printer to make this part. Pay attention to use material strong enough to survive possible strong forces.