Tuning bandpas filter with thin film dielectric glued on top, what is that material?

Long time ago i saw amateur website, where band pass filter tuned a little down in frequency by adding thin film of yellow color above the filter. I tried using pcb dielectric and plastic film from advertisement, but it is difficult to glue. So any suggestions on that material?

The thin yellow film was most likely Kapton take.

Thank you! It is what i am looking for!

yeah but kapton tape is not very high dielectric constant. A small piece of ceramic, rtv-ed in the corners, may work better.

also, depending on the filter type, u might get by with adding solder, or even short lengths of wire, to the resonator ends. A solder blob, with its fringing capacitance to ground, will tune a resonator lower in frequency.