You have a few options.

A methyl cellulose solution works well and you can use it for larvae as well as embryos: https://zfin.org/zf_info/zfbook/chapt4/4.4.html

A glycerol solution will work the same way, I usually use ~60%.

Also see this past conversation: https://www.researchgate.net/post/How_best_to_immobilise_zebrafish_larvae_for_imaging

Thank you very much! it's really useful for me.