I simply drilled holes through a piece of pallet wood for the battery mount, then bolted through both onto the rack, held on by aluminium chipboard joiner plates and nyloc nuts on the underside. I used aluminium joiner plates because the rack is aluminium, didn't want steel wearing it away. Has been pretty sturdy, for about four years so far. The plan was to replace pallet wood with a bit of aluminium box section, bought but haven't bothered. I could move the battery closer to the middle of the bike, for better weight distribution, for my uses that hasn't been necessary. Using velcro straps over the top to secure the battery, is a good idea - I've got two also holding on two waterproof rucksack covers.