In the middle of my delta printer build (custom variant of the Mini Kossel) I realized that I was running out of t-slot inserts. Looking to buy some I either had to pay way too much for the delivery, or buy them from China and wait for 2 weeks. No fun. What I do have a lot of are common nuts and washers, so I made my own inserts instead. Works like a charm! The M3 nut and M4 washer (dia. 8.9mm) snaps right in and takes all of the axial load, the printed parts just makes the insert easy to mount and prevents the nut from turning. I use 2020 extrusions with 5 mm slots from Motedis, but I am sure these can fit similar extrusions as well. Remember to print the inserts either with the nut+washer side facing upwards, or standing up. The .stl is not oriented correctly, will fix this soon.. Pretty small part, so a reasonably good calibrated printer is ofc preferred. I would also recommend printing them at pretty fine settings. 30 mm/s at 0.15 mm layers was a good mix between quality and speed on my printrbot simple metal (4 minutes / insert).