aloha kawika...interesting question and dead on the money...i guess in the true sense unless the subject soundboard is made of manmade controllable materials the soundboard consistency will always be suspect.

problem with plexi as a constant would be the need for amplification to generate and register changes in sound volumne and amptitude when testing for variables like tone bar dimensions and placements.also the inherent stiffness of the plexi might make the tone bar variables hard to pick-up.

think personally to test soundboard variable...baox can be consistent; meaning sides and backs and neck etc.

only variable is top....

then many tops at same species and thicknesses...test prior for sound and or stiffness (deflection) with weight and micrometer etc.

amass consistent stiffnesses. use these as test boards.
cut to exact sizes to fit premade box.


start with one constant..ie  bar patterns  couple same, couple different...or bar heights couple series tall other short etc.....install tops..test  remove , record and try again......sounds like a lot of work but i am positive the learning curve will be quicker than making a hundered or so ukes with no purpose in mind

anyhow just my thoughts     laters  ukeman

aloha kawika....

Asa is spot on....use aluminum thin sheets for a consistent soundboard with acoustical properties...relatively cheap, consistent and very acoustical, prepare for a wide smile. Should be a great testing soundboard material..around 1/16th thick should be cool....hope this helps   ukeman