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8-day bump.
3 month bump.
4 month bump.
And another bump.
It wasn't Willy and The Cardboard Boxes? [1]
Damn, the ones I keep turning up are all too recent. Lots of books about kids playing with boxes! Well, it is a trope, after all...
Or the "Great Big Box Book"? [1]
You're right, that's not it. Like you said, those are too recent.
There were no flights of fancy ("He builds a plane...") or anything like that. And it wasn't an instruction manual like the GBBB.
FWIW it was probably from Scholastic Book Services.
Edited by randomsurfer
Yet another bump.
Read in the 1970s.
Picture Book about a boy who has a cardboard box toy. All his friends want to play in/with him in it so he fortifies it so that they can't get in. Then he gets lonely, and realizes how much he enjoyed making the fort so he goes into a cardboard-box-fort making business.
Edited by randomsurfer