Black PVC pipe is very cheap. You should be able to find the right size for the job. That should make your setup look more "professional" than the pepsi cup.
teleconverter ? .. I did not use any teleconverter .. I use a Canon closeup lens 250D (my dc no macro mode and need it to take macro) and use focal length from 200mm to 370mm to give bigger magnification ...
as for barrel distortion , it is visible at all corners ... tt y I take the shot of the slides with some space / allowance ..