GS boxes are already valved...I dont know how "open" wouldnt be loud enough given the mods you cite...
That said, if you want to gut a pair of boxes..Id do it to a pair of 4200 boxes rather than ruin the standard Tubi made originals..
There's a pair of GS boxes for @ £1K on eBay...
or a pair of tatty 4200 boxes for a couple hundred quid..
Glad Im not your neighbour...!