I feed all my gals in their cages. I got AP T10s and Pro Line cages. There is no advantage to feeding outside the cage, especially as the snake gets bigger. All you are doing is causing more stress to the snake and yourself. if you are worried about the snake eating substrate, just use a paper plate, piece of newspaper or cardboard. I personally just feed everyone on aspen except when they were babies. I then just used little paper plates or sheets of printer paper. Now as they are larger, i'm not worried about them eating a few pieces of aspen. Their stomachs can digest bone, I don't think a couple pieces of shredded aspen will hurt them.
If the snake makes a mess while feeding, I just wait until they done and a few hours later, I just spot clean the area and wipe it down with F10 and add new aspen to fill in the spot. By then the snakes don't care what I do as they are full and usually curled up inside their hide or curled up in the back corner under their shelves.