A sock would come off sketchier to me than a journal that seems like it actually sees use. Just say who you are and who you play and ask them if they'd like to play a PSL with the two characters because you really enjoy their take on X and would like to play with them sometime. Have a basic scenario in mind but don't get overly detailed right off the bat (it will probably be easier to springboard from an idea based off the canon, assuming both your characters are canonmates). If they say no thanks, take it graciously.