If the film can be sanded down easily then that's good news! Thanks for the info bro.
I dont suggest lining the entire length of the box walls with foam, that will add mass to the internal volume and if the added mass wasnt taken into consideration when designing the box then it will alter the alignment, in other words mawawala sa tono ang kahon.
It might give an illusion of better bass but basically it's just bloated lows and at the expense of other frequency range mostly in mid highs to highs. Been there, done that
The film is very thin, sanded [exposing the wood] the portion to be joined. Roughened the whole interior surface before application of lining. I'm experimenting only with different enclosures sizes & configuration. I work in a construction, have unlimited supplies of form plywood on site .