There was a time not long ago when APPIC had few to no restrictions on what sites could request as supplemental materials. Sites began to demand more and more, and the application process became even more unreasonable than it already was. Some training directors were requesting individualized recommendations, additional essays, multiple essays, work samples, and more. I think this was partially so that sites could try to lower the number of applications they received by making the process even more demanding than necessary. (It must be terrible receiving so many applications that you have trouble choosing which of the overly-qualified candidates you will choose.)
The year that I applied to internship for the first time, APPIC mercifully put their proverbial foot down and announced that the only supplemental materials allowed are 1) a sample redacted assessment report and 2) a work sample/case conceptualization. Luckily for you, you probably already have these! I guarantee that, by the time you apply for internship, you have written at least one work sample for class, and if you are applying to the kinds of sites that require a sample assessment, you have had a practicum experience at which you have written several to choose from. Just double-check with your supervisor that you can use these reports.
With your supplementals, make absolutely certain that you have removed any and all identifying information about the client before you submit them. The many urban legend-horror stories about internship applications include several tales of solid applicants who were rejected because they accidentally left the client’s name on their redacted assessment. This means remove first and last names, addresses, home towns, et cetera (I left the state that the client lived in and that did not seem to be a problem). Refer to your client by first initial only, or by a fake name and clearly state that the name has been changed.
I also recommend having more than one sample for each of your supplemental pieces and choosing based on the type of site requesting it. I had two assessments: one of a young child whose primary issues were behavioral, and one of an adult with severe developmental difficulties. As usual, you should have extra sets of eyes look for style, format, and typos.
Good luck! Your application is almost complete!