Adapt your standard walk-in with an extension that mixes the chilled air from the box with the room temperature air outside, making the temperature within the extension to be at or close to cellar temperature (low 50s is ideal).

Cut a hole in the side of the walk-in cooler, and install a sliding ventilator in the hole. Build a fixed plywood extension as shown, long enough to accomodate casks (however many will fit or you want space for) with at least a foot added to the length to accomodate the tap and hose. Height should also be a little more than the size of the cask to accomodate tilting, spile pegs, and cask breather spigots (if used). Also take into account stillaging, if to be used rather than having casks resting on the ground.

Build a free-standing, fully detachable lid to cover the new area - use two pieces of plywood permanently attached to each other at a 90 degree angle. Cut a hole in the front of the cover and attach a sliding vent, as with the vent on the side of the walk-in cooler. Cut a small hole in the top of the cover for feeding the beer line(s) up to the bar, this can also be used to feed through a cask breather line from the gas supply. Attach a handle to the top for ease of lifting. When the set-up is complete, the beer lines should be covered in insulating foam on the complete run between the box and the beer engine. Also, it is a good idea to cover the inside surfaces of the extension with a layer of insulation material.

Resting the casks on portable stillages ('cradles') is recommended for maximum stability (take this into account when measuring as the casks will be three or four inches above the ground). Regulate the temperature inside by opening and closing the vents as necessary - placing a digital (or other) thermometer inside the box is highly recommended for attaining perfect cellar temperature.

Cask measurements for guidance (firkin):