For the shear webs at the lower end I've used leftover perspex, and the same off-cut I have used to cap the underside of the accommodation booth. Then in common with a past practise I've bolstered the inside of the base with 1" Celotex insulation board of a type more often used to insulate your home.
Around the inner edges of the top and bottom ends of the booth you can run a bead of silicone, which both helps fix the edging and dampens vibrations at the same time.
