Maybe too small for double wall! no RC1 or RC2 in area!

Hey everyone, So basically i will start by saying my mistake and how i'm planning to fix it...thats the part i need help with! Initially I build a vocal booth by quickly searching random internet sources on google and ended up building a pentagon vocal booth with two layers on sheetrocks (4 inch of space in between) and filled the space in between with bedding foam, two layers of thick glass (28''x28'' each on one side) and one of the most mediocre doors ever (2" thick with 2"x2" lumbers for framing, 1/4 plywood on each side and bedding foam in between). The interior size of the studio is 4'x4.1'x4.1'x3.7'x3.7'. Then i read Rod Gervais' Book that opened my eyes and got me dissapointed for a whole weekend! Now my problem is this I would have wanted to keep a single wall structure, replace the bedding foam by fiberglass or rockwool and put resilient channels before inserting back the inside layer of sheetrocks (5/8). However there are no resilient channels in my area... and having it shipped to my area would cost a fortune and would take a very long time. Therefore I wouldnt mind doing the double wall structure, however im wondering if its going to be too small ONLY in terms of SOUND QUALITY but not space (its only going to be one singer in there anyways so the space is no problem for me but im worry that the sound quality will drastically decrease). Air space would be 1.5'' and both layers are about 4' thick with an additional 2 inches of rockwool panels on the inside PLEASE HELP!! Thank you very much!!!!