I can't tell you with this particular pgm but can in general.
You created the arch for the curve of the top for the front and back correct?
Take those two vectors (the front curve arch and back curve arch)
you need to measure the width of the arch's, then measure the cavities width.
take the cavity width and cut your vectors off the ends of the arch's width and that should give you the proper curve for making the the 3d cutting of the cavity, if that makes sense.....
HOWEVER.....if the top metal base of your wheel rollers that attaches to the board are flat (which i bet they are...) then you don't need to do all that.
What i think you need to do is take the shape of the wheel base and carve that into the bottom of the board. Let us know what that is, i'd be curious.
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Just remember...when it's time for the hearse to pull up..there's no luggage rack on top!
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