There are some good ideas on this page (that I hadn’t remember reading the last time I was there so maybe they updated it) that involve a 4′ x 12′ section of culvert and sandbags. This is something that could possibly be stored in advance even if unused and assembled within a 24 hour period by a family. The link suggests inside a garage.
My concern for the 4′ culvert plan is that it might turn into a sauna in a the warmer seasons.
The military guidelines they mostly copied in these links are generally better, more honest and clearer for a SHTF environment than the civilian ones.
(The only caveat is, Keep in mind however that military manuals are based on the resiliency of a young adult male.)
As for a rad shelter:
Any location can be a rad shelter as long as its big enough.
Keep to the middle of a medium or large house, for 3 days.
Generally this means whatever dose rate was outside has decayed to 1/1000th of it previous rate which will restore your freedom of movement outside in all but the most extreme cases.
I wasnt looking at blast protection just fallout protection from the title of your post.
I am pretty comfortable with the subject as I have been doing, or teaching it for a long time :)
There is nothing wrong with your plan and every little bit of shielding helps.
If your house is largish staying in the middle of it, or middle of basement, will give you very good protection though as any Fallout will be at distance (windows/walls) from you.
3 days = 10 half lives or more for the high activity isotopes, which is a reduction factor of 1000 :)
It’s getting late and i have a long drive tommorrow.
I’ll talk about it in a more nuanced way probably Sunday nite.