I had some cut-offs from doing the same thing, making deeps into mediums. I use mine when I feed syrup with ziplock baggies. some, I cut in half and added a new side piece on so they fit my five frame nucs. I've got some on now.
Baggies are great, but your hives have to be fairly level, and I've learned that if you make your slits closer to the top of the bag, you can change it out without having to wait until they get every bit of syrup out. With the slits closer to the top end, you can carefully lift it up and collect the left-over syrup in the bottom part of the bag and pour it into your new bag before you lay it down.