too much cubic ft air space

I have an enclosure with too much air space thus making my sub move tooooo much. How can i remove some of the air space? I cannot cut the enclosure or anything as it is upholstered with vinyl and i dont wanna mess with that. Can i use a expanding foam? Or is the only way too box off some of the insides with mdf to take up space... If foam is a good option what type of foam should i use? Sub is a Diamond D6 12" Box is vented.

I did the expanding foam and it sucked, the sub of course moves a hell of alot less but it barely creates any bass i think perhaps my port is too big or the foam just absorbed the sound waves. Im gunna try the styrofoam blocks then if those dont work ill buid a new box :(

how much airspace does the box have?  if the box is built with a port already and you lower the amount of air volume, you change the tuning of the box drastically and can be putting the box freq out range of the woofer or what the crossover point is set at.  you need solid objects to reduce airspace because porous materials absorb backwaves and in turn make the box sound bigger.  too much airspace in a box may work to your advantage if you want a good sounding sub because a bigger box will usually give a better transient response, sounding smoother.  if your woofer is moving too much reduce power.  you could use it as a low velocity style box and that can give you big bass from small amounts of power."If a man made it, another can modify it...it just takes some thinking."

"If you ask questions, you're a fool for 5 minutes; if you don't, you're a fool for a lifetime."