I remember that the boy spent his own money to buy the kit.
He built a castle and they were able to explore within the magic world they created.
This is NOT Edith Nesbit’s "Magic City"

I read this book 15-20 years ago, so it must be from the 80′s or 90′s or earlier.

Not everyone likes to be given a list of links to look at, (No matter how relevant they are). If you would like me to delete the answer, then contact me and I will, no hard feelings.

These search sites will help you find it.

http://www.auntbook.com/

http://www.whatsthatbook.com/

http://www.librarything.com/groups/namethatbook

http://www.loganberrybooks.com/stump.html

http://forums.abebooks.com/abesleuthcom

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