I thought one of the "draws" of Holga's WERE the light leaks.
A cheap solution is using opaque tape on the OUTSIDE -- after you figure out where the offensive leaks are -- but it looks like you tried that and don't like it.
There are thin, sticky-on-one-side, black, foam sheets that you can buy -- in various thicknesses -- but depending on your camera, they may be more expensive than the camera, and not that easy to apply.
Another idea is that there are lots on inexpensive cameras with "Holga" lenses that DON'T leak -- and often have more features. True, they lack that snappy cache of HOLGA, but life is full of tough decisions.