What about the default profile that Guix keeps in ~/.guix-profile?
You can assign it the role you want. Typically you would install the manifest of the packages you want to use all the time.
Alternatively, you could keep it “manifest-less” for throw-away packages that you would just use for a couple of days. This way makes it convenient to run
guix install package-foo guix upgrade package-bar
without having to specify the path to a profile.