Resources

To 3D print your own Bahtinov mask for accurate manual focusing, I suggest starting with AstroJargon's incredibly useful Bahtinov Mask Generator (now a standalone Windows app, check out the README for instructions on use), which will generate a SVG file, you can then import that SVG file in to tinkercad, add a collar, and download as an STL file for 3D printing. For 3D printing, either find a 3D printer at your local library or university, or if you can't find anything locally you can use 3d Hubs to shop around for the best price. Here is an example of a Bahtinov mask I made for a Skywatcher 80mm doublet refractor:
Here is another one I made for my Canon 200mm f/2.8 L ii (note: the spacing is very tight; probably won't be able to print this in PLA; I went with vinyl):