I have been doing this for a while by following a pattern found here[0]. I typically follow this and copy every asdf package dependency into the working directory of whatever project I'm working on [setting the subdir-search-registry to (list (truename "./asdf-systems/")) and placing the files into a subdirectory of that name] .This way I have the option to make changes in them without affecting anything else on the system depending on the same package. In general this means I use Quicklisp as a (huge) convenience in finding packages when programming interactively in the REPL, but not in deployment. I think this setup is also nice because it is dead simple to drop in copies of asdf packages from the web that aren't found in the Quicklisp library.
[0] https://en.wikibooks.org/wiki/Common_Lisp/External_libraries...