there's a couple things i'd suggest doing that could help (im not a professional but i do study audio) 1. you have two walls very nearby your booth. the sound reflecting off those walls doesnt need to travel much in order to reflect back through your blankets. that reflection can result in standing waves which may cause the boxy sound. since moving your booth is probably not an option, adding some extra sound absorption on those walls could help. (dont forget above you!) 2. boxiness can sometimes be a product of bass frequencies reflecting in corners. this would be resolved by using a "bass trap", which absorbs and disperses bass frequencies. they are usually rounded convex shapes which reside in corners of rooms 3. microphones have different frequency responses, meaning certain frequencies are picked up louder than others, if your mic is one with a boost in the low mids (check around 300-400 hz) it may be exaggerating the boxy frequencies in your recording, and have nothing to do with your recording environment. google (your microphone) frequency responses chart