Bought a couple more spools of PETG 3D printer filament, one was a standard 1Kg size (blue) the other a 2.5Kg spool (black) which offered a big saving compared to a 1Kg spool. I should have expected it but the physical size of the heavy spool took me a little by surprise! There was no way it would have been mountable on the printer had I not modified it to use external spools, the spool rollers I made sit on a shelf above the printer.
Home made spool rollers
One big spool of filament
Edit: A cool idea I just had - one of the problems with many filaments is they are hygroscopic and need to be kept dry, they are transported in a plastic bag with dessicant inside. Of course, as soon as you take the spool out the bag it will start to deteriorate unless you take steps to prevent it (construct a dry box for example). With the spool rollers I figured I could just put the roller inside a large sealable bag, with dessicant, and the filament then pokes out through a tiny hole. It can be used this way and will stay dry. Seems to work fine!