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Scaling parts that have to fit together with a specific clearance like this is tricky. If you scale it up by 150%, it also scales the gap between the two parts up by 150%, so it's too lose. And in the other direction, if you scale down by 50%, the gap is 50% as side, and it might be too tight. You'd have to do some math to work out exact percentages, but I'd suggest scaling the outer part slightly less than the inner part so that the clearance between the two is constant.

Or you could use one of the parametric designs for this that have been here on Thingiverse, like http://www.thingiverse.com/thing:6004/ from 2011, and use it to generate exactly the size that you want.

A-Mazing Box Detour