I would caution you strongly against using 16% or 22% gel in a tray
that is not custom made.  The potential for burning the soft tissues of
your mouth is too great.
    Furthermore, these carbamyl peroxide preparations are chemically
unstable and have a short shelf life.  So you'll either risk burns or
risk a suboptimal result.
    I think you're better off with the Crest White strips if you're doing
it yourself.  Much safer.

Steve