Found used 5 sided Vocalbooth. Need help with placement

I have located a used 5 Karat Diamond Platinum (double wall) series Vocalbooth for sale, and need some advice on placement and acoustics.

Vocalbooth says the unit is designed to fit into a corner, but I'm thinking I need to pull it away from the corner somewhat to avoid the dreaded third leaf effect. How far away from the adjacent walls should I place it, and should I put rockwool in the gap between the wall and the booth? My goal is to keep as much outside traffic noise and low frequency rumble out of the booth as possible. Will I need bass traps (and are the foam ones any good?) Would it be better to use something like fabric covered rockwool or OC 703 instead of the foam on the inside for absorption?

Many thanks! Greg Thomas http://www.DeepWarmVoice.com