I used 1 inch perforated steel tubing from ( home depot or loses or similar place) 3/8 th threaded rod cut to the length needed ( mine was 8 pieces 8 inches long) for the carrier I used 1.5 angle 1/8 thick ( I would have preferred alum. but was expensive here) cut to size , I needed 4 pieces 6 inches long to clear the battery, two pieces the depth of the box and one piece for front and back of box. Bolted together with 5/16 bolts and ny-lock nuts and flat washers, the box I got from costco it's a tradesman 21, I had to alter the lock to suit my set-up. It took longer to plan than it did to put it all together, hope this helps as I did not take pictures as I was putting it together.
"Well that didn't go as expected"
2015 Chev 2500HD Highcountry Duramax
Cedar Creek Silverback 33IK
Donald&Casey cairn terrier
Rest in Peace Mary my darling wife.
Scottish by birth Canadian by time.