Downspout Guard

You only need to print the "Drill jig" and  “Mesh cut template” parts once. You'll use them as tools.
For each downspout onto which you want to attach one of these, you need 2 parts, one of each collar (inner & outer).
Installation requires drilling a 1/4" hole and inserting a 1/4"-20 screw with at least 3/4" of thread to secure the guard to the downspout.
I used a 316 stainless steel mesh (1/4" openings) and the way the parts fit together require that the mesh be no more than about 1mm thick. I got the mesh from Amazon.com: (https://www.amazon.com/dp/B0CRZ1DCY8?th=1). This mesh is easy to cut with tin snips.
No supports are required for printing, if printed in the orientation shown in the images.

Installation: NOTE the steel mesh is very sharp and will cut you. BE CAREFUL!

Prepare the gutter's downspout first:
1) insert the red cross into the downspout with the printing facing out of the opening
2) slide the "Collar - inner" part onto the downspout. It should easily slide on until the flange touches the face of the downspout's opening. This is your drill guide.
3) inside the downspout, pull the red cross ("drill jig" part) toward you until it touches the flange of the "Collar - inner" part. This lines up the clearance holes on the drill jig with the holes on the collar. The purpose of the drill jig is to support the thin aluminum of the downspout from below, when you drill through it.
4) pick one of the holes on the perimeter of the collar as the one that will have the screw that holds the assembly in place and insert your 1/4" drill into that hole and holding the red cross in place with one hand, drill through one side of the downspout. (although designed so that the drill harmlessly goes into the drill jig's hole, be careful NOT to drill your hand!)
5) remove the parts from the downspout in order to assemble the entire gutter guard. set the drill jig aside to use in other installations.

Assembly: (wear protective gloves any time you're working with the steel mesh)
1) lay the "Mesh cut template" part on the mesh, on a sturdy bench or table. Use it as a guide to cut the mesh around it. Use tin snips to make the cut.
2) after cutting the mesh, place the "Collar - inner" part on the mesh, centering it on the mesh. 3) fold the MIDDLE of the mesh up, on each side of the part up, holding the part down to help make a sharp fold.
4) continue to bend the folds, working outward from the middle of each side and moving toward each corner. There will be a little excess material at each corner, just like when you wrap gift boxes. (see image)
5) you may want to trim the excess material, using the tin snips but I have been successful assembling the parts without this trim step. (see image)
6) place the "Collar - inner" part on the formed mesh, flanged end up (the opposite of the way it was printed)
7) place the pre-folded mesh over the "Collar - inner" part, sliding it down until the mesh rests on the flanged face.
8) place the "Collar - outer" part on top of the assembly oriented the same way the other collar is oriented and press it firmly down over the mesh and inner collar. This traps the mesh in the assembly so that no critter can push it out. Keep pressing down on the outer collar until the holes on its side align with the holes on the inner collar.
9) verify that the mesh does not block any of the holes in the sides of the collar by looking into each hole. If the mesh can be seen though those holes, remove the mesh and inner collar from the outer collar by pushing the mesh and cut away the area that was blocking any of the four side holes.
10) once the side holes are all clear of mesh, reassemble everything as you did in steps 6 & 7

Installation:
1) slide the assembly back onto the downspout the way you slid the inner collar over the downspout for drilling.
2) when the flange meets the end of the downspout the hole you drilled in the downspout will be lined up with one of the holes in the assembly.
3) screw a 1/4"-20 screw into that hole in the outer collar. The side holes in the outer collar are smaller than in the inner collar. There are no threads for the screw in the hole. The screw will deform the PLA, creating the threads.
4) tighten the screw until the head is flush with the side of the outer collar.
5) looking through the end of the downspout, verify the screw can be seen protruding into the interior of the downspout. If it doesn't, get a longer screw and try again. The specified thread length for the screw should assure penetration into the downspout. This locks the assembly onto the downspout.

Check occasionally for debris trapped in the downspout. Remove the screw and pull the guard off the downspout to clear out debris and reinstall.