You can see under the subwoofer the timber base..just 20mm ply
Had to TIG weld in 30mm square Hollow Section SHS on both sides of the rails..took alot of time as I milled the rivets out and then needed to put a slitting saw thru the welds--then set up a jig to weld all the sections back in place so that the mounts all lined up and the rails still slid easily.
good luck..remember its a safety device so suggest dont do any shortcuts