A premium solution is the Grin BOB product - https://ebikes.ca/shop/electric-bicycle-parts/battery-accessories/triple-bob.html
It provides not only a rock solid flat platform to mount your battery to, it also moves the battery mounting points up the downtube a few inches. You can use it with or without the additional tube clamps.
In my case I used it to mount a big 52v Reention case battery (the long 385mm one). The offset moved the battery up the down tube just enough for some needed clearance between the bottom of the battery and the seat tube. I used it with 3 water bottle bolts to the frame and 3 bolts to the batteries mounting plate and didn't bother with the additional clamps.
Grin also offers a two bolt version (but is currently out of stock). There are also other similar options on the market, but I haven't seen on build exactly like the Grin product (with the "outriggers" that allow the plate to mount without rocking).
