I was thinking of sandwiching the ply, mdf and acrylic...
I would substitute Richlite for the acrylic, the material is very dense and naturally non-resonant. Richlite is made from layers of recycled cardboard or cellulose fiber and phenolic resin.
https://www.richlite.com/ The photo below showing the layers is from the sample kit, Richlite is impossible to break and would never look like this if you could break it.

Richlite is dense, a 6"x6" square of 1" Richlite will weigh 1.6 lbs.
My unscientific resonance test:
https://www.youtube.com/watch?v=SkSxH40x5SAI used Richlite to dampen speaker cabinets and it worked very, very well.
https://www.audiocircle.com/index.php?topic=72808.msg683847#msg683847The 3/4" and 1" Richlite pieces weighed an average of 1.25 lbs. each

Look for sink cutouts from countertop installers otherwise it can get expensive. Cutting Richlite requires a triple chip blade and a really powerful table saw, I used my 5hp, 3 phase, 12" Powermatic and it was slow going.