Best practice for including custom .deb in Debian preseed

I have a bootable Debian 11 USB drive, and I'm writing a preseed configuration file to be loading at installation time. I've written a custom app.deb (not the actual name), and my goal is to have it installed at the time of system install. I've been trying to figure it out for several days, and it seems like there are potentially multiple methods for doing this. Is there a best practice way for loading binaries through a preseeding process? I want the binary installer to be local to the drive - I don't want to have to set up a repository.