We are very particular about how these go together because we see so many broken and cracked weld battery hold downs, and most of the broken ones weren't cheap either. The real problem we have found with the easily broken ones was multiple welds on too thin material, some are only 20 gauge, and they simply can't take the vibration.
We followed the form of our single battery boxes and made these with very close tolerances so they didn't have to be too thick of material because if it fits well it won't be bouncing around. So we use 12 gauge (about 1/10" thick) and MIG the center tab, that should avoid the vibration cracks around the welds the rest are so famous for. And here they are lengthwise.
Each battery has a hold down in the shape of the newer battery top (if you have an older Optimas they will still fit). We do not pre-drill any holes for mounting because you will want to mount them for YOUR vehicle and not have to work around someone else's idea of what is right. They can be bolted or welded in but remember that batteries are heavy.