Neat solar filter for telephoto DSLR lenses. IMHO, looks far better than cardboard crafts.

Step 1: Materials and Tools

Materials

ABS filament in case of 3D-printing OR 3 sheets of FR-4 plastic (or similar), 100 × 100 mm.
AstroSolar® Safety Film OD 5.0 or it's alternative. OD 3.8 is OK for photography too but must not be used for direct visual observations.
58 mm screw-thread lens mount. I took my from a cheap replica of Mennon DV-S 58.
Three M4×6 mm bolts (or similar) with nuts.
Cyanoacrylate glue ("Super Glue").

Tools

3D-printer OR fret-saw.
Knife.
Scissors.
Q-tips.
Sandpaper of different grades (coarse and fine).

Step 2: Preparing the Parts

The idea is to put the solar safety film in a flange frame that can be mounted to different adapters. Adapters can be made for DSLR, telescope, binocular. In this instructable we'll make a DSLR adapter.

You can either 3D-print the parts, or cut them from a piece of plastic. 1 mm FR-4 sheet would be good, but you can use pretty much any material you have. I chose 3D printing just to check it out (my first printed thing). The material should be UV and heat resistant as it will be exposed to direct sunlight.

Use attached 3D models as a reference.

Cut (or print) the parts. Sandpaper them if needed to remove the artifacts. Drill ø 4 mm holes for bolts. Pins on the circle flange are optional and used to superpose the parts together easily.

Step 3: Film

Cut a ø 70 mm disk of AstroSolar® Safety Film. 60 mm is the aperture of the filter + 2 × 5 mm overlap for gluing. Do not remove the protective paper from the film until you're ready to glue it.

Step 4: Filter

Put the glue on the inner surface of a circle flange and glue the film on it. Do not forget to remove the protective paper! Act expeditiously as the glue hardens very fast. Do not stretch the film!

Wait 1-2 minutes till the glue hardens and cover the flange with a glue again. Put the second flange circle so that the pins fit the holes.

Put the filter under some pressure for a while.

Step 5: Adapter

You'll need a thread mount compatible with your lens. In my case, the lens has a 58 mm filter threading, so I need a 58 mm mount. The easiest way I thought of is to order a cheap lens hood on AliExpress and cut the threading mount from it with a sharp thin knife.

Sandpaper the cut until it is smooth and parallel to the second, original, face.

Glue the thread mount to a "triangle" flange part — it will be the adapter between 58 mm threading and the filter. I used baking soda as a filler for super glue, but I do not recommend it. The result is unpredictable and looks poor. Some soda crystals were not "melted" in the glue and fused in the resulting polymer.

Put the adapter under the pressure for a while.