Finally got myself a heat gun and tried the above "hat" method. Didn't work as PETG plastic doesn't really stick to itself fast enough not to burn it.
Also tried this ( link posted before as well):http://www.youtube.com/watch?v=LFEvctCEYq8
The problem again is a petg plastic. Impossible to stretch so much like in this plastic barrel. Which is interesting since, as you can see at the above photo, a vacuum did work but wasn't perfect coz of escaped heat.
But now I do have a method to heat a plastic and than use it on a vac table. Basically using the same barrel with a plastic as on the above youtube example, but heating from below with a heat gun through a hole.
Than placing upside down the barrel on a vac table might work. Will try that soon.
This eliminates a need for big heat oven. It might also keep the plastic hot enough while vacuuming.
I guess it really depends now on how to vac it and how does it stretch without many wrinkles.