Yep and you said "¾" or 1" birch for "normal" boxes" and "for walls 1" MDF might be better"
Normal boxes people use 3/4" MDF with the front double baffled and braces. Works just fine, people have been using that kind of method for years and years with good results. If you can afford Birch more power to you. Yeah it is a nicer wood no doubt but not everyone can afford it and in some areas like mine it costs a whole hell of a lot more then 3/4" MDF does. Either way you are gluing it together & sealing it.
When you are doing a wall build combined with multiple amplifiers, batteries and all that ground & power wire it adds up really quickly.
Not too mention the weight of all the subwoofers. Wall builds can easily go to 1000 pounds.