what i started with..

and how it ended up.. (i had 15mm on each side to fit cladding and insulation, 10mm stand off batons 3mm marine ply, and bubblewrap/mylar diy composite layer insulation ) it didnt help being quite windy when insulating.. (back wall, floor, and ceiling have 50mm polystyrene layers..)

Im having a 'cant see wood for the trees' moment this end.. So forgive me asking but rather than run a shelf chiller 24/7 and maintain cold kegs wouldnt it be easier to simply use the chiller as intended? I have found that for my needs a maxi 310 hold sufficient cold in its waterbath at 0C without any ice wall as im never serving at a city pub rush hour (consecutive pints) rate.. So it only takes 30 minutes or so to chill down the waterbath ready to serve an evenings pint or 4..
If your under the stairs position is closed in i would keep a very close eye on things, I have found that if i run my chiller mid summer with the door closed on the beer cellar (ex coalshed/ washing machine cupboard) it can raise the internal temp to 40C in no time at all due to the chillers heat exhaust.