Deluxe Framer
Level the frame, draw the 4:5 box, get the numbers. Your picture stays on this device.
Tap the left end of something that should be level (a podium edge, a countertop).
Drag the box to move it. Drag the corner dot, or use the slider, to resize. It stays 4:5.
How to get the frame onto the phone (Termux)
Grab the still at the moment you want (seconds into the video), saved to Downloads where this page can open it. Replace the video ID, the time and the name:
ffmpeg -y -loglevel error -ss 1234 -i "$(~/bin/yt-dlp -f 'bv*[height<=1080]' -g https://www.youtube.com/watch?v=VIDEOID | head -1)" -frames:v 1 ~/storage/downloads/NAME-frame.png
The numbers this page gives are in the frame's real pixels: angle for rotate=-ANGLE*PI/180, then crop=W:H:X:Y on the leveled picture, as in the Remote Systems manual, section 9, step 2.