Need some help designing a small vocal booth

I am building a small vocal booth from scratch. I have read hundreds of posts over the past two weeks. I think I have a really good handle on isolation, but I am still unsure about some other elements of the design. (The construction will be from double steel studs spaced by 1" with two layers of 5/8" DW both sides and fiberglass insulation.) First question: I am not clear how to use the various room mode calculators to ascertain ideal room dimension ratios. My booth will be approximately 9.5'H x 7'W x 8'L, but I have some flexibility to tweek those a little bit. I found some published ratios on the sites of both Auralex and Ethan Winer, but none of them match my approx. dimensions very closely. My ceiling height is greater relative to the other two dimensions than anything on those tables. Do I need to drop another ceiling to lower the ceiling height in order to use one of the published ratios? Are there unpublished ratios which would come close to matching my dimensions which I could determine using a room mode calculator (or some kind person could calculate for me)? Or should I just not worry about the ratios and fix the problems with acoustic treatment? Second question: (probably should have been the first question) Is there an advantage to making the room pentagonally shaped, or would I be better off with a four-sided room with walls not quite parallel? If I choose the four-sided option, how far from parallel do the walls need to be? Third question: For this size/shape of booth, what acoustic treatment should I start off with? I expect to have to do fine tuning after everything is built, but it would be good to install some basic elements to begin with. I have seen some good plans from Phil Cramer for a Deep Bass Trap, a Mid/High Absorber, and a High-Bass Trap. I have also looked at the Corner Absorber, the Side Wall Absorber, and the Rear Wall Absorber plans on this site. Also Auralex foam, and of course OC703/705 covered with fabric. I thought I would use some combination of these, but I keep hearing differing opinions about what would be best. Since the room will be used mostly for vocals, will the very low frequencies be a problem? Lastly, I would like to say that I am truly impressed with the amount of expertise I have found on this site, and especially the willingness to share it. Thanks to everyone who has contributed all the amazing and incredibly useful information.

you could take this idea and resize it to fit. i'd say in the larger booth, you could use slats on the corner to add angles and then add porous absorbers as needed.