Design for Guitar Booth, please review

I have a home studio in my basement. The total room is approx. 18' x 21'. I would like to build a guitar booth for recording and sound isolation purposes. The attached drawing shows dimensions. I will probably put one layer of 5/8" gypsum board on the inside and outside of my 2 x 4 studs with 2" rockwool inside. My inside measurements are roughly 8' high x 7'10" wide x 10'4" long. I cannot go any wider or higher. Please let me know if there are better ways to do this. Thanks My specific question: Should I drop the ceiling height some to be have a better ratio with the width or will it matter for this size room? I will be recording mostly loud guitars and some vocals. I already have 20-30 panel bass traps to install where needed.

i'd consider dropping the inner ceiling to 7'4" to give you a better set of room modes. what is the slat diffuser? if its something like this attachment, i'd put them over the cover traps. plan on some porous absorbers on the ceiling to break the ceiling to floor reflections.

yes, those are the exact slats I found to build. thanks for those. I thought dropping the roof would help and yes, I have broadband panel absorbers for the ceiling. thanks for the info.