I already had pre-build static/opevpl cingg in

/dev/shm/new-git/cinelerra/cinelerra-5.1

so I cd there

cd tools

wget https://github.com/AppImage/AppImageKit/releases/download/continuous/appimagetool-i686.AppImage

(note, for x86_64 name will be different!)

 chmod +x appimagetool-i686.AppImage

cd ../

guest@slax:/dev/shm/new-git/cinelerra/cinelerra-5.1$ setarch i686 ./bld_appimage.sh

on x86_64 you obviously can skip setarch part

it was a bit scary because it re-run configure with its own parameters, but at the end it just sailed past build phase and packaged resultin binaries in cinelerra-5.1/bin as cin-i386.AppImage

so, in general procedure works, I think, just make sure you have makesquashfs installed ?