Does velcro really hold the boxes down?
I use wire ties and attach them onto the board itself, mainly cos i don't tweak my pedal setup much and its easier to clean than carpet.
Allergic to dust, so i'd stay away from carpet.
What I did was drill holes into a piece of plywood where the ties would go, then set the stuff down and tie them.
DIYing is fun.
Good luck on finding your carpet. And car audio shops are a good place to start. Or else, you might wanna try office furnishing shops.