Actually, for someone like the OP (who plans to film without entering search zone) a good alternative *might* be to have a good sound recorder in a pocket. That way, if there is a request to delete evidence, and if the person says "Would you please confirm that you are requesting me to delete evidence?", any positive confirmation would be captured. It might even be sufficient to lead to prosecution of the LEO.

(This is not legal advice because it might, or might not, be asserted as a violation of the two-party consent laws in some areas, although I would readily argue that a LEO, with knowledge that taping has taken place, standing in a public place, making an official request, does not have a "reasonable expectation of privacy" and further, there are exceptions under many laws even in California.)