Print in something stronger (and impact resistant), I used ASA but PETG should also work. Install a hex key into the appropriate insert, push into the cutout in the shaft, make sure it sits all the way down, then put the retainer ring over it, orienting its cutout to fit over the protruding bit of the hex key, and whack it on. I found this works best with a table vice with slightly open jaws, place the holder so that the hex key is in the slot and hammer it on. Don't overdo it. Use a soft hammer like a rubber mallet and increase the force until it goes all the way. It is a very strong connection and won't come apart. I had to replace one of the keys and the only way I could get the ring was to break and peel off the retainer. Clean the end plug from any stringing or blobs (particularly the point may have some), put some silicon grease, silicon oil or something else slippery on if you have it around (it's not strictly necessary so don't worry if you don't). and snap the end cap in the shaft, you're done. Mark the end caps with the key size. I used a label printer to print it out, stick it on, and then covered it with several coats of clear nail polish (letting every coat to dry for about a day), and it came out great.

Except for the inserts, all the parts are identical. If you find that the keys are not sitting firmly in the inserts, you can dribble some glue (thin superglue e.g) into them to secure them.

Print out the stand if you wish - it only holds 5 pieces because that's all I needed (the whole set contains 6). The stand is designed to use little space, unfortunately it is also somewhat unstable and can be fiddly to pull out and put back the screwdrivers.

Use the holders like you would use the electronics screwdrivers - have the end cap sit in your palm and twist the shaft with your fingers. For high speed twist on the narrow part of the shaft, for higher force use the thicker parts, for even more force (if you wish to destroy the bolt or break off your key's ball end XD) you can also use the protruding part of the hex key as a lever, but this is not what this is meant for. If you need force, use the part protruding out of the side in the screw and the holder as a T-handle. I don't recommending applying force beyond the thinner part of the shaft, but otherwise, they are surprisingly strong and will give you quite a bit of leverage.

I use keys with the ball end, because this is the most flexible and high strength is not required in this application. You can use whatever you wish - including Torx keys. That's not what this is designed for, but it will work, provided you are able to fit them in.