Hey Mathéo!
As far as magic arms go for monitor mounting, my favorite is the ball-clamp style from SmallRig that uses rubber o-rings to ensure that the arm itself doesn't move under inertial stress, not even a millimeter. Some other generic magic arms can work fine for this, but they tend to rotate loose.
There are decent rubber feet on this one to prevent the 1/4" ends from slipping or rotating with inertial moves, but the NATO rail and double-pin versions can completely solve that issue if it does become one with a heavier monitor. It should be noted, for these solutions I'm using an Atomos Shinobi SDI (discontinued model) or a BlackMagic Video Assist 5" for my low-mode monitor. Anything heavier may add vibrations or shake loose more easily.
It's also worth noting that I'm using either a battery eliminator or DC to D-Tap cable to power the low mode monitor to both reduce weight and battery-swapping needs. No reason to have batteries on that thing.
SmallRig Ballhead Arms:
1/4" to 1/4" Ballhead Clamp Magic Arm
Version with double pins for ARRI
Version with NATO rail clamp (can be helpful to use a NATO rail on the low-mode monitor or the camera cage itself, further removes the ability for the monitor and arm to accidentally twist off with inertia)
Double length version (still stiff, but more leverage on the mounting points, so consider swapping the ends for NATO clamps to ensure it won't shake loose)
As far as where or what to mount it to, I typically mount a low mode monitor off a 15mm rod clamp with some cheese holes, like this one: https://www.amazon.com/SMALLRIG-Railblock-Thread-Camera-Shoulder/dp/B075M52W52/
This is a perfect block to add a NATO rail to as well, so that any quick pans or other moves won't twist the magic arm loose.
Here's this solution seen on my old Zephyr rig:
You can also ditch the arm entirely and just mount the monitor right to the upside-down rail block with a small monitor ballhead. The ones for coldshoe mounts, like this one, can be used if you unscrew the shoe and use a longer 1/4" screw: https://www.amazon.com/Adapter-Cameras-Camcorders-Smartphone-Microphone/dp/B0811X28XY/
Honestly, I don't typically need to add this rail block though, as there's usually a spare 1/4" hole to mount the arm or a NATO rail to that's already exposed somewhere near the front of the camera stage.
Here we were using top rods, so I mounted it to the closest 1/4" which was on the side of the RED's baseplate, which is off-center, but the double-length ball arm was able to swing around and re-center the monitor.
Here's a build using a simple clamp to hold the low-mode monitor on using the double-length version of the ball clamp arm.
I don't remember why I didn't use the rail block for this one--maybe so I could reposition it easier or to off-set the balance of the motors--but it worked fine!
If mounting it on rods is not possible, and there's no other cheese plate holes or NATO rails to conveniently mount the monitor to, the next best thing is to look for any mounting points on your sled or baseplate. I'm not familiar with the Archer 2, but the M-2 doesn't have many unfortunately. I've had success a few times by bolting the 1/4" into the baseplate itself, but it definitely can come loose easier. There's also a rod mount for the front of my M-2 top stage which would be perfect to add two stubby rods to for low mode monitoring, but I haven't found the right off-brand threaded rods and don't feel like forking more money over to Tiffen right now for some overpriced piece of metal, lol.
I'm curious to see any other operators' low mode mounting methods!