Going to liquid nails the collar into place tonight, so that I can get cracking on the tiling tomorrow. Very nervous about this - the tiles on the side of the collar will be very close to sitting proud of the fridge. If I get the collar wrong by even 1mm, it will be out on each side. Front and back will be fine, because if anything, I made the collar slightly too narrow on these sides so there's plenty of room.
I've been searching for suitable insulation for what feels like days and I've finally found a good solution. Most of my enquiries were about products that were either too expensive or you couldn't get in small orders. I've opted for the TradeSelect Thermal Break 8, which is an 8mm foil insulation filled with XPE foam. I got a 1350mmx1000mm sheet for $18, which will be enough to put 2 layers on each inside face of the collar (16mm total insulation).
Coming together now!