Built a cajon recently.
I tend to research stuff.
After fishing around I realized that all resonating boxes with holes in them are Helmholz resonators.
These include acoustic guitar bodies, djembes, kora's, bass reflex speakers, coke bottles, marimba resonators and speaker cabs.
Some oke did some experiments:
http://34iac.acoustics.sk/proceedings/Kicak.pdf
The box resonates with a fundamental bass frequency.
The hole size affects the frequency and the volume of this resonating fundamental note:
- increasing the hole size increases the frequency; and increases the volume ... to a point
- volume of resonance is maximised with holes in range of 150mm to 220mm (6"-8.5") depending on volume of box
- open back is over the top ... and less loud than some optimal size
All of this is in respect of the bass reflex effect.
There's obviously the separate effect of hearing the speaker directly.
For my cajon I found if the hole was too small the bass response sounded "choked".
Making it bigger made it "free-er" and louder.
Until it got too big then you had a very different effect altogether.
So ...
No idea how this actually applies to cab speakers, but if I were you I would explore 3 options:
a) closed (which is where you started)
b) open (which is easy)
c) holes ranging from say 6" - 9"
The material doesn't make much difference providing it is rigid.
The bass comes from the air resonating; not the walls.
Absorbent walls can make a difference by absorbing sound ... but that's another completely different effect.
If I were you I'd make a back out of some mdf or ply and cut a 9" hole in it.
I'd then make a few squares that can be placed over the hole with smaller circles in them.
Say 8", 7" and 6"
Then you can play it and dynamically experiment by putting the "squares with smaller holes" over the big hole to see the effect.
If you're in the mood for experimenting.