In custom filter allow to generate list of words ONLY containing provided letters.

[Hooch180]

I want to practice a set of words that ONLY contain the letters provided. A use case for that is to, for example, test new switches or keycaps by replacing only a few keys on the keyboard and generating a list of words containing only those letters.
I know I can add a list of excluded letters that are all alphabet letters except the ones in the include. But it gets tedious the do it manually.