how to remove cu.ft from sub box?

So I built a fiberglass sub box for my 350z to fit under the strut bar knowing it would be too big. But, now I gotta figure out a way to remove .4 cu. ft. I could obviously just fill the box up with wood blocks but I'm not trying to make a boat anchor. So instead I was thinking I could just put some triple expanding foam in there. Not sure if this will work to effectively decrease the volume of the box though? Need some help.

The foam will work great for what you are looking to accomplish.  It will reduce the interior volume while keeping the weight down.  All that matters to the sub(s) is the net air volume inside the box.-Dual XHD6425 - HD Radio w/USB & 3.5mm in

-Power Acoustik PS4-840

-Infinity Components

-10" Clarion SW1051 in FG molded box

even lighter would be somr sealed air tight containers, like you put your sandwich in, just glue the lid down