I made a custom box, and as of now, the only thing holding it down is the carpet [Which isn't really that good]

I can't screw down into the trunk [from my knowledge] because the gas tank is under it, and I'm not sure how far down it is.

I was thinking about using straps, but that seems bad.

I was also thinking of using 90 degree angle metal pieces to screw it right into the back seat

(My box is set up to go flush with the rear seats so when you bring them down, you can see the subs)


Best picture I have of it

 

if it is flush with the back seat then try and use some L brackets to secure it to the area around the back seat. If you can't find any that work then make some. Get some thick weldable steel from the hardware store and bend them to what ever shape you want via a big hammer and a vise. I made the brackets that my box hangs from under the rear deck this way. I think there is a pic of them on my cardomain page.