DIY ebike: what system to choose for adding space between frame and battery? So, the project is to install this 5kg battery:

on this bike:

This is no joke. I'm serious.
Following up to this previous question, I'm going to use steel rivnuts (aluminium is really too light for the job), M6 if possible otherwise I'll fallback to the standard M5. I hope to have enough room to put at least 6 rivnuts, 4 being the absolute minimum I could accept.
Here are some dimensions of the rivnuts:

Now the point of this question. The frame has an hinge, thus I cannot install the battery right on the frame, there has to be a spacing of about 15mm between the frame and the battery. The battery goes from the fork to the seatpost, with just a few centimeters left, allowing to swap the battery out from its support.
So far, I could think of 3 solutions:
1) Use these steel hexagonal studs:


Pros: Visually, seems to be the &quot;right piece for the job&quot;. Made of steel.
Cons: Probably not designed for such job, I'm afraid it could break between the thread and the nut part.
For the record, M5 model has 8mm thread length and M6 model has 10mm thread length.

2) Use long bolts and these &quot;tall washers&quot;, between the rivnut collars and the battery support:

Pros: The most &quot;regular&quot; system one would think of.
Cons: Probably not the sturdiest system.


3) Use larger &quot;tall washers&quot;, that would sit around the rivnuts collars, right on the frame, rather than on the rivnut collar.

Pros: Allows to use much larger, thus stronger, washers. For example, with M6 rivnuts I would use M12 washers, that have an external diameter of 20mm! That would help a lot for bearing the sideways force.
Cons: Maybe need to add a protection layer between the frame and the washers, so they don't leave a mark on the frame. Maybe a band of Gorilla Tape, with holes for the bolts.

So far, I'm bending towards solution 3. What are your thoughts on this?
I think a problem with option 3 is that the screw’s tightening torque is “pulling” the rivnut out of the frame (in addition to any force acting on the battery). With option 2 the frame only receives the actual forces acting on the battery.
Sideways stability should come from your mounts being several centimeters apart, not from using wide washers/spacers.
In all of this I’d make sure to use threadlocker and sufficient tightening torque (like &gt;4Nm) to make sure nothing comes loose and starts weakening over time.
Oh, and one idea: Instead of rivnuts, why not drill through the whole frame and use very long bolts and a bracing plate (or wide washers) on the top and bottom? Or even just clamp plates around the frame without any drilling and mount the battery to the plates.