Sammys 8003957-25 Vertical Rod Anchor Super Screw with 1/4 in. Threaded Rod Fitting, 1/4 x 2'' Screw, for Wood (Pkg.=25) , Zinc

I would imagine you could suspend a sub using strut, all thread and a sammy screw / anchor, but you're going to have to be willing to modify the box so you could get some sort of bolt to secure it to the strut. https://www.amazon.com/Everflow-SAMMYS-8003957-25-Vertical-Anchor/dp/B074ZCCBK3/ref=asc_df_B074ZCCBK3?tag=bingshoppinga-20&linkCode=df0&hvadid=80058400530448&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=79264&hvtargid=pla-4583657879743486&psc=1&msclkid=d26fbbaa698f1bd64dccd18251824d28