Ahh! Another Ender 5 I see? Is that the Pro, or base model? I have an Ender 5 Pro myself, have had it now for almost 3 years. I started with a Prusa clone (RepRap Guru) which was made from acrylic panels, wiring, and motors. Had to put every little screw in the bastard, and then calibrate it for another month before anything "cool" came off of it.
Boy, then I got the Ender 5...pull the machine out of the box, lube up some spots, level the bed, and away we go. I couldn't believe how simple the setup was.
Anyway, Solo SIPs! You have a decent DIY design, but in my humble opinion, the little solo cups don't need all of that. They don't need a fill tube, and don't need an outlet tube either. Again, just my opinion. With the outlet hole positioned correctly, all you should have to do is dip the cup into another bowl/cup of water and let it fill to just above the hole (I use clear plastic cups so I can see where the water goes). Then it goes into a red solo cup to cover up the roots that fill the sides.
Here's what I came up with, but haven't tested fully. I started some catnip with this prototype, but things got busy and it fell to the wayside. I'd still like to test this design, and will definitely be using it to start some canna seeds in a few months once my current grow gets wrapped up.
In the second picture you can see the outlet hole (or overflow if you call it that) and it's located 3/4" below the top of the 3D printed insert. If you dip the entire assembly (with soil of course) and let the water flow through the overflow hole, I can quickly see when it's watered. Takes 3-4 seconds per cup usually. Anything extra flows out of the overflow hole and back into the main water cup/bowl. This setup negates the need for an official fill tube, which makes transplanting easier.
While this design looks neat and provides the same functionality as the commercial gro-inserts for the 5 gallon buckets, it's completely un-tested and I'm not convinced you'd be able to remove the roots from this little insert without some stunting damage. Once they grow through the top mesh and down into the 'res', it'll be a pain to remove it. Perhaps leaving it through the entire grow, or changing the design entirely, I have no idea (yet).
Anyway, this is super long, sorry to hijhack your thread! I created a 3D printing in the Grow Room thread a while back and have posted some stuff there if you want to check it out. (Link in signature). I'd be happy to trade STLs though, and/or work on developing a design for stuff. The idea of sending someone a virtual file that they can then print and functionally use is one of the main reasons I started printing many moons ago. Really cool stuff!