Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4015227.
This is my stab at a flying extruder using the stock motor. I'm using a gear-reduced dual-drive (Bondtech BMG clone) extruder, so the stock motor is serious overkill. If you have a pancake motor 23mm or less, you'll be more interested in my other flying extruder design.
I originally designed this so a simple 3mm cap-screw could be used in each of the holes to attach surgical tubing (with a zip tie) going to the carriages. Then I found these mini-bungie cords at Home Depot, so I made the bungie-loop to mount them. They turned out to be the perfect length. The position of the holes for the bungies (whatever type you end up using) was as close as I could get to evenly distributing the mass among the 3 carriages. This does make the rear bungie a bit loose (if you use the ones mentioned above), so I simply tied a knot in the cord behind the metal hook to shorten it up (if you see the design of those bungies, this will become obvious). The reason for the row of holes along the diagonal top beam is I wanted a bit of flexibility with a counterweight. I saw someone post elsewhere (sorry, forgot where) about a nice (professional-quality) retractable key-ring that has plenty of pull (made for a huge keyring) called a "KEY-BAK SUPER48". I got the 13oz model since I'm using the heavy stock extruder. I actually haven't installed it yet as the stock motors actually hold this setup when unpowered (which I didn't expect).
After installing this, I dropped my retractions all the way to 2mm. After switching the bowden tube to a 1.9mm ID one, I was able to drop my retractions to 0.4mm! The cube in the pictures was printed in 3D Solutech See Through Blue PETG. The two things that helped the most were this flying extruder setup and my corner reinforcements.