Why I chose this position:

This vertical placement is best because it maintains a factory ride height and mounts to the frame (not handlebars).

Downsides:

front is heavier and wheelies require more effort, battery and controller are designed be flat under the deck so hopefully they take shocks as well in this position. Less easy to jump at the skatepark.

Next step:

I’ll make an upsidedown “T” shaped mount from a combination of aluminum scrap and possibly some 1/4” plywood to mount this permanently. Then I’ll add exterior protection and enclose it in a camelback.

To build a 2wd M365 with all factory parts like mine you'll need:
battery $54
controller $30
motor, tire, tube $50
stem $10
circuit board and cover $25
segway es2 foot brake $12
data cable $5
grommets $5
brake handle $8

total=$199 in Ebay parts
.......+$120 on Craigslist for this scooter with a malfunctioning headlight
__________
grand total=$319

You could buy 2 brand new Xiaomi M365s for $400 each and have all of the parts (minus $12 Segway ES2 foot brake)