I think the best way to carry heavy sla batteries is to build a cheap easy trailer. So here is the most basic design I have come up with after much experimenting with them.
To attach the square tube to the bike I use a couple of pipe clamps. You can drill holes through the tongue part of the tubing since that part does not have any rods running through it.
An "L" bracket is attached to the bike at the rear axle. I weld a 3/8 bolt into the trailer tongue so that I don't lose it. That and a couple of nuts make a quick attach and release. I grind off the excess of the L bracket so that It doesn't trip me. None of this damages the bike. You can put an old inner tube between the L bracket and the bike if you are concerned about scratches on the paint.
