The QR codes are just silly bureaucratic arcana and not some high tech & secure validation of the test result. They are just a url to a web accessible or downloadable version of the test report (you can scan yours with an app like Google Lens if you need to download a copy of your report). I haven't seen anyone every check the code anyway, they just look to make sure the report has a code & never bother to load it.

If I had a test result with no QR code & was in a hurry and did not have time to get another test, I would.
1. Find the public URL to the report (usually sent to you in the results email), if there was no url, then I would upload a publicly accessible version of the report to Google Drive and use that url.
2. Use a website like https://qr-creator.com/url.php to generate the QR code
3. Paste that QR code in a blank spot on the report and reprint it. Or if I was feeling some qualms about altering the report, print the QR code on a separate sheet of paper with an official looking caption and title. something like "Scan to validate covid test report dated....."