Neck placement, box width

Hi everyone

I'm thinking about building a wood box for my cardboard dulcimer.

Does the neck have to go down the middle, or can it be offset a little, to allow some unique woodgrain to show?  I'm not sure how that would throw off playability, if it would be harder to balance across my lap, or do weird things to the tone.  

Also, is there a formula as to proportion, how wide the box is vs neck width?  

This isn't going to be a thing of great beauty, just something better than cardboard for the time being.

Thanks, Lisa