Back in the mid-20th century Popular Mechanix Illustrated had a story about ganging flat styrofoam speakers, designed for waterproof installs in boats, in an array that apparently gave good sound quality.
This is a deep dive into using flat sheet materials with an audio driver.
The $30 cost is subject to inflation, according to the comments. He shows how to off-center the driver and add tuning nodes. I doubt end-grain balsa is cheap.
"Not as good as my $1000 speakers". Subwoofer still required.
The interesting thing, to me, was that one of the top three materials is 'pallet board/card', a hex-cell paper board. I can't find a source but I'm hoarding a foot square sample.