Someone on another forum bought a Powermax 3" soft plyo box and attached it to their 20x24x30 wood plyo box with velcro.
Here's the link to the powermax soft plyo boxes: http://www.powermaxequipment.com/sto...soft-plyoboxes
Not sure if I can link other forums, so I'm just taking the quote:
"I ordered this 3 inch high density foam plyo box top to retrofit onto some of the stuff at the gym. I really couldn't justify $1000+ for a full set of soft plyo boxes. I am going to have it set up to work with the wood plyo boxes. Bought a roll of velcro at Ace Hardware. Total investment with velcro and pad is like $150."