I'm using transparent film with soft adhesive used in sign making/print shops. I'm in this kind of business and had the opportunity to do some testing.
I found this:
http://www.orafol.de/eng/pdf/620.pdf to do a great job.
You can get 1 sq. meter for 5 bucks/euros or less which could be a lifetime supply.
This kind of material can be found at most manufacturers worldwide. It should be thin (less than 0.1 mm), transparent & clear, with removable/ultraremovable adhesive, adhesive power less than 5 N/25mm. For ex. maybe a better choice would have been
http://www.orafol.de/eng/pdf/1610.pdf with 1.5 N/25mm but I couldn't get it, I ordered a sample and i'll have it in a couple of weeks.
All you have to do is cut pieces 62 x 82 mm from this material (which i cut on a cutter-plotter)
You can also check with the sign making/print shops in your area and you might get pieces/samples for free.
Good luck!