Done it many times - it's pretty easy as long as you change every instance of blank_sidebox in each of the files to the same new name.

From the installation readme in the mod files:

Adding Several New Sideboxes

If you're adding more than one new sidebox, you will need to do some renaming to avoid clashes. Simply replace all instances of blank_sidebox and BLANK_SIDEBOX in the files and the filenames with your preferred name e.g. logo_sidebox and LOGO_SIDEBOX.