I believe they come with "L" brackets, or at least they used to. When I had mine installed in my 80 and 100 series, I sourced the correct bolt in a tapered allen head and mounted them to the third row seat mounts. The thread pitch and bolt size is a very uncommon one and I was lucky enough to get in on a group buy on 'Mud. I just drilled the four holes in the bottom of each box to match the OEM holes. The result was a very clean install with no unsightly "L" brackets. There are pictures of the whole setup if you search.
Originally Posted by Flagster
2003 7.3 PowerStroke E-350 EB
2006 Jeep Unlimited, AEV Highline
1957 Land Rover Series One, 107"
2011 Toyota 4Runner (Hers)
1980 HJ45 Diesel LBP (Sold to some dude in Utah)
1984 Troopy, ExPo White (Sold to the same dude in Utah that bought my other dream truck)
Kimberly Kamper (Sold to the Toyota Shaman)
2001 7.3 F-350, 6 Speed w/FWC Camper (Sold)