I finally was able to cover the gaps in the vocal booth door. I used rock wool and moving blankets. no more light is getting through the door, but it is by no means air tight.
I was a bit disappointed that adding plywood only added -2 db to my noise level . I guess in the future, I will need to invest in a whisper room or something similar. Although my noise floor is acceptable for audiobook standards, I'd rather have it at least -75db. Studio specifications were tested from a raw audio sample sent to an audio engineer.
Studio specs before:
Levels: No problem here at all, peaks about -7dB is perfect
Noise: -63dB RMS room tone is good, and it sounds clean and quiet.
Mic technique: Very good! Full, rich, clear, present with minimal plosives (heard 1 or 2 so move mic 2" up to reduce them).
Acoustics: GREAT, the space is dry with no echo or resonances.
Studio specs after:
Recording levels: Spot on for a raw audio sample
Noise floor: -65dB RMS is quite acceptable, no issues here
Mic technique: Excellent, focused, clear, no plosives. The mic is very bright and sibilant sensitive, so EQ in software will be useful to compensate. At 51 seconds you move further away from the mic, so be consistent as possible.
Acoustics: No issues when proper mic placement is observed.
While I didn't get the results I was looking for, I am really proud of the work I did in creating the booth and am excited to start recording more audiobooks in the new year.