Hold down your sub...ideas and pics please!

[B-chi]

The only safe way to secure a sub box is to screw it down properly. Velcro won't atop it flying through the rear seats in a crash.

[Snoopy]

Exacty. Watch crash tests and read about the forces involved do you really think a box of ~2 stone will be held down well with velco?

[Bespoke Division]

I have also seen some very scary videos.

Best way would be to make a false floor mount that to the boot floor then mount the box to the floor.
The safest way imo 

[B-chi]

I have also seen some very scary videos.

Best way would be to make a false floor mount that to the boot floor then mount the box to the floor.
The safest way imo 

The only way. My sub and box weigh 70kgs. That's no different than a person in the back not strapped in.

[Snoopy]

I have seen the mess MDF makes with the forces too. It cannot handle the G and often splits when used as false floors and supports. So any mounting hardware now is steel or ally for me. With proper bolted mountings.