I find that putting a bowl of boiling water into the freezer usually gets it going fast enough. I have seen sprays on sale for defrosting but I don't know what they leave behind or if the speed is worth the cost.
unplug the freezer.
put a couple of towels in the bottom of the freezer put a washing up bowl on top and fill a third of the way with boiling water and close the door apart from a few inches.
keep changing the water and use a wooden spoon or spatula to help remove the ice.
do you know anyone who has a wallpaper steamer?? might sound odd but my old Earlex came with a nozzle attachment for a jet of high pressure steam. It dissolves the ice within just a few minutes. I actually defrosted mine today..first time for ages & it was caked in ice. 15 minutes & it was switched back on.