The OP design might see a big jump in effectiveness if they add an air guide one one or both sides of the box fan (just make a simple square cardboard tube with some tape) so that by the time the air reaches the filter, it has laminar flow. This helped enormously in a DIY design I kludged together in Thailand (where box fans are NOT available, so I had to get something to work with regular round desk fans, which everyone already has):
This may also explain why the OP heard that the tape "helps" but was surprised when it didn't. The key issue is likely not sealing with tape but rather forming an air guide.
https://slice-of-thai.com/air/
"cannon" design (ugly but effective): https://slice-of-thai.com/air/#p3
This may also explain why the OP heard that the tape "helps" but was surprised when it didn't. The key issue is likely not sealing with tape but rather forming an air guide.