I guess any tips that are passed on will be the same for any cigar box instrument.
Have a plan. This may be a full working set of scale or full sized plans, or at least a really good idea of what, and how you are going to build your latest creation. Depending on the style you wish to achieve, will it look as if the whole instrument has been thought out, or thrown together in an afternoon in your kitchen? Both styles (and many more) are viable.
With a bass you will need to appreciate the strength that will be needed in order to carry the tension of the strings. Will it be a two, three, four or even five strings.
May I suggest a 'through the body' neck in order to take the string tension. What scale length will you use?
In my opinion a full scale length will be better, but the longer the neck- the stronger it will need to be. Concentrate on the neck (will it have frets or will it be fretless?), the fingerboard (if it will have one) and also the joint (S) to affix the neck to the body. Make sure that and electrics for pickups etc. will fit into the body of the cigar box once the neck has been fitted.
Be sure that there is enough room for the string tuners (machine heads) to be fitted to the headstock.
I hope that these really basic suggestions give you ideas for your masterpiece and steers you away from any basic errors that you may not have thought about.