We tried a lot of techniques and I have never find one which was really fully reproducible - one major problem is not only bonding plastic substrates to PDMS, but also to keep it bonded with water or cell culture medium. You could try:
Gluing with uncured PDMS:
10.1021/ac062118p
Silanization (in order from the most recent to the oldest):
10.1039/C5LC00285K
10.1063/1.4883075
10.1039/C3LC51324F
10.1039/c0lc00272k
10.1039/b924816a
10.1039/b820924c
Or simply sandwiching your PET substrate between partially cured PDMS parts:
10.1038/s41598-020-58507-4
I have a feeling that adhesion between a PDMS part and anything else also depends on the mold used for soft lithography, and using 3D-printed molds might reduce that adhesion compared to SU8 or micromilled brass...