In this sense base64 does 80% of the work, if plus and solidus are unwanted, you can delete them from the result by hand, another option is head -c 20 /dev/ura

In this sense base64 does 80% of the work, if plus and solidus are unwanted, you can delete them from the result by hand, another option is head -c 20 /dev/urandom | md5sum