I was thinking about this as a mental exercise, but mine would be indoors (security, dry, relatively dust free, stores on hand etc). It would be created from supplies easily and cheaply bought from local builders merchants at short notice or what I have in the garden already.

In essence, seal the windows, vents etc in whole house as best as possible. Choose a room, sandbag windows in the chosen room to create first wall of mass depth i.e. surrounded by sandbag or brick. Within that room, create a double hull shell made of plywood, fixed to a wall for stability. Fill the hull space with soil from garden to required width (so that you have 30 inches plus of mass between you and the outside world). Within this ‘soil hull’ build a strong inner frame of sleepers (already on property) with ply roof. Put soil (or sandbags) on roof of structure to required depth. Small door to shelter would be sand bags (sealed as you enter for the final time) fitted with crude air filtration system. This would not be a blast shelter but fallout only (and I appreciate my description of the shelter is outline only). I think even my basic DIY skills combined with stress would make this doable in a few days. It is effectively a room within a room. Brick walls obviously.