This project was born when I fitted an e-bike conversion kit to my other half's bike and realised that the concept of using mounts designed to hold a small water bottle weighing less than 1kg to support a large battery of 3-4kg weight, especially if used on uneven tracks was a recipe for damage to and possible failure of the mounts and frame.
Simply put, it's a method of using custom 3d printed saddles matched to the bike downtube to stabilise the battery and to add additional secure mounting points independent of the bottle mounts.
Two saddles can be used in conjunction with the bottle mounts simply to stop sideways movement of the battery. For more security, extra saddles with provision for a hose clamp and captured nut add the extra strength. The pictures show the basic concept and the setup as applied to our bike.
On a bike with round downtubes, this is an easy design process, and a single saddle design will suit for all supports.
Where it gets complicated is when the downtube is not exactly round or even worse, tapers and changes profile over the battery tray length. I have run into this in three more conversions. In these cases, the profile of the tube at one or more points has to be measured and transferred to the 3D design software.
There are several methods to do this, but I settled on the rather crude option of using an inexpensive finger profile gauge to take an impression of the downtube. I then put a piece of 1mm ruled graph paper on the back of the guage, scanned it into an image file, scaled it way up and printed it. I was then able to read off the co-ordinates to be used to construct a splined curve in the 3D design software. Then print a small trial profile, test and tweak as necessary, then generate the final saddle, possibly leaving some slack for rubber padding between saddle and frame (old bike inner tube).
Another alternative is not to try to match the profile at all, but simply have two points of contact spaced either side of the centre line. I have put an STL file for this option in the Files section.
The saddle depth at the attachment point needs to be deep enough to include the thickness of a stainless steel hose clamp and captive nyloc nut and at least one exposed thread of the bolt used without digging into the downtube. I usually assemble the battery tray and saddles before fitting it so I can make sure that the bolts don't protrude past the saddle. You need to drill a 5.5mm hole through the stainless steel hose clamp in the appropriate position. *Make sure that the clamp is well secured while drilling so you don't rip fingers off and use a sharp drill*.
When fitting, use old bike inner tube material between the hose clamp and frame so you don't mark it.
The saddles were printed using PLA with 1.2mm walls and 80% infill. So far the ones fitted to MTB cycles have held up very well. PETG is probably a better choice.
I have included a 3d print STL file to suit a 44mm diameter downtube that can be scaled up within reason to larger sizes, or to act as an example when you are designing your own saddles.
Alternatives: You could try the Double Or Triple Bob mounting system by "Grin" and for extra security, add hose clamps: Do a web search on "ebike battery Double Bob" for details.