I use Faultless hot iron cleaner.....says it even works on gunk from fuseables...I do not use fuseables so no experience with that.
but between now and when you can go to a store....
If you have a generous sized piece of muslin or an old pillowcase you can place it on top of your rectangles and then iron...... that will protect the rectangles but will really mess up the press cloth, so be sure it is not something valuable.