I thought I would avoid the temptation and show the thought behind designing a map now.
The Map: A traditional 'Corner Shop'
I solved this through a mix of experience, and the handy use of Google Maps. Simply by comparing the Google Map image of the building, with a similar building I already knew the measurements on. Not an exact science but good enough to work with.
The internal fixtures (the shelves, the freezers, the chiller cabinets etc) were all sourced from the internet. A brief search for 'shop fittings', 'shop display' etc came back with enough companies to browse through. All the items listed on the websites had the exact measurements. After this it was just a case of a small bit of maths to convert those numbers into my drawing scale. The end result, although not a fantastic piece of art, was something close to the item with the correct measurements.
I intend for most locations, especially important ones such as this, to have at least a couple of variants. A ransacked shop, that other Survivors have already hit for example, as well as a uber rare 'fully stocked' version.
Once the drawing phase is complete, the next step is to incorporate the location into the game and work on the 'Loot tables'. This is easy for the most part, its simply listing items you would find on the shelf of any shop and marking their benefit (i.e. +10 to Food). Where it gets complicated is adding 'Rare' Loot, items of real high value. And making Loot tables for ransacked shops. Something useful may be left behind but what exactly?
Thanks for reading
PS. I have uploaded the map to the game pool now. There are no rules for it yet, but you can still have fun battling the Diseased across a new location!