I assembled a small ukulele from a kit and it needed brackets to hold the sides in the correct shape while gluing.  I had a piece of MDF for the base but needed brackets to brace the sides.  I quickly drew these up, printed them and they worked great!  There are rectangular ones for the ends and curved ones for the middle “waist” of the instrument.  You'll need two of each so plan accordingly.  I was originally thinking double stick tape would work, but for my kit, there was enough pressure from the side panels that the curved side brackets needed to be screwed down and the rectangular end brackets had to be clamped to the base board. 

Printed in PLA but anything should work.