Not that I have a underseat sub, but wouldnt some velco work?
something like this..
https://www.ebay.co.uk/itm/Self-Adhesive-Sticky-Back-Hook-Loop-VELCRO-Brand-Tape-1m-Hook-and-1m-Loop-/180854389807?pt=UK_Crafts_Sewing_Supplies_MJ&var=&hash=item2a1bc2fc2f