This design is for an ender3 side spool mount (yes, another one) but this one is with a twist, it incorporates a weight sensor, so with the octoprint plugin "Filament scale" you'll be able to know how much filament you have left, from the octoprint web interface, in real time and with precision.
So, what you have to do is to print the two parts in the STL, and from there, you need to locate 2 M5 machine screws, and 2 M4 machine screws. I dug through my ender3 spare screws that were leftovers from the making of it, and found the right screws, so no need to buy some. To link the printed support to the stock spool holder, you reuse the stock screws and nuts (the rectangle 10x10 extrusion nuts)
Then, you need a weight sensor (they're called load cells) and found mine here: https://www.aliexpress.com/item/32958628401.html I bought the 5kg load cell with the HX711 AD module.
Mount all this toghether, and connect the load cell to the HX711, and then the HX711 to the raspberry pi. WARNING: the instructions provided here : https://tutorials-raspberrypi.com/digital-raspberry-pi-scale-weight-sensor-hx711/ are great, BUT, they had the GPIO pins wrong for the CLK and DTA pins (+5v and GND are fine).
You need to connect the CLK and DTA pins to GPIO 20 and 21, see here for the right pins (took me a while to figure that simple thing out) : https://www.raspberrypi.org/documentation/usage/gpio/
And, then, all you have to do is go to the Filament scale plugin, it will tell you if the connection is good or not, and if so, you have to tare and then put a known weight to calibrate the scale (don't forget to press SAVE, don't ask) and then you can weight an empty spool, and you then get the current weight of remaining filament, live on your octoprint page. Neat!