mounting battery on full suspension bikes

So I have a rear rack battery 48v 20ah the kind in the aluminum case that fit on those handy slide rails.

first off, yea should have gone triangle bag, was thinking that but the aluminum case just seemed safer when I was shopping, and i like the idea of being able to lock the battery to the frame

anyway, I have a full suspension bike
https://www.bicyclebluebook.com/SearchListingDetail.aspx?id=35230&make=694&model=45845

so I'm just tying it down to my rear rack now but I was thinking its a shame not to use the rear shock for the battery.

would taking a seat post rack and maybe adding arms to attach to the rear shock arm where it pivots on the down tube . . . hmm is that gonna work?

or let me hear from all you folks out there who have rattled around with a batt on a rear rack for years with no problem

in a bag with minimal padding now I might just add a yoga mat and bolt the slide rail mount on top of the pad (better more dense foam ideas?)

There are a few topics about this if you search around. For sure having your battery bouncing around without benefit of the rear suspension is asking for early failure and possibly a fire. Mounting in the triangle is ideal for that frame style. Seat post mounted racks will tend to fail pretty quickly with the weight of a battery, but are the right idea. A reinforced seat post mounted rack might work. The rack needs to be triangulated back to the frame somewhere below where it attaches to the seat post.

Options..
1).. Sell the rigid pack and buy a suitable triangle pack with bag.
2) ..put the rigid pack in a backpack ...many pros and cons for this method,..but especially good for off road.
3) ..strip and rebuild the rigid pack into a triangle frame pack..if you have the skills.
4).. Search and find a suitable frame mounted rack to suit your rigid pack. Likely expensive with worst handling outcome.
So many choices !

For a build on a DH frame with a hub motor, the best balance is achieved mounting the battery on the front of the DH fork. Alu mounting plate and protection receptacle that is fitted to stanchions with 4 clamps between steering crowns, a quick release battery bag is inserted in the alu receptacle, and a binder strap holding it firmly. Of course this does limit battery size and most who are doing this kind of mount are using RC lipo and relatively short range battery. Additional battery can be fitted to top tube or down tube for extra range, or carrying a spare battery in a backpack.

You can get sheets of practically any foam online if you want something with a higher density. I've used here before for a different project. Same principle though.

If you have to carry much weight on a seatpost rack, whatever the heavy thing might be, this is how you reinforce one.
Add struts from the rack back to the bottom of the seatpost. After installing them, scrunch the seat down a tiny bit to snug it up really tight on the strut, and the rack will wiggle a lot less, helping handling.

But the bottom line is still this, if your battery weighs more than 10 pounds, any kind of serious off road riding will suck with all that weight back there. I found an 8 pound battery back on the rack, and another 8 pounds on the handlebars helped, even though it was lame compared to a mid mount. My bikes can't carry a triangle mount battery, since they are old fashioned Y frame bikes.

for rough trail /off road riding , you seriously should try the backpack method.
Taking that weight off the frame makes handling in the tricky bits so much easier.
And much less vibration on the battery.