Here is a simple DIY planner for 2021: 2021-DIY-Planner.pdf.
If you have access to a printer that can print on both sides of a piece of paper, you can print yourself a letter sized, two pages per week, 2021 planner using this file. You can then hole punch the pages and put the planner in a binder.
To create this, I used an awk program to write PostScript for the main planner and ps2pdf to convert the PostScript into a PDF. The initial calendar pages came from a combination of another awk program and the cal program.