Resources

To 3D print your own Bahtinov mask for accurate manual focusing, I suggest this online tool SkEye's Bahtinov Mask Generator, which has a 3D output option built-in so you can skip using TinkerCAD to create the collar. Instructions: see the Help section on the website for most options. Change output type to 3D. If you want a collar that loosely goes around the end of your dew shield or lens hood, use the Flange Height option and set it to something like 20mm.
If you don't own a 3d printer, you may be able to find a 3D printer at your local library/university/makerspace, 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):