5 psi chamber pressure won't get you to 2.4 ATA of inspired oxygen. Be really careful with building a chamber yourself. A fire inside a pressure chamber is a horrible way to die. Asphyxiation and explosive decompression are also risks.

Correct, 2.4 specifically is 35.27 PSI. I was just suggesting I can start with a small amount of pressure to test it myself. Avoiding explosions is rather simple. The air in the chamber can be normal levels of O2, which just means I need a proper mask that does not leak the O2 or the air I exhale. I already have O2 sensors. I can just tell it to alarm at 25% O2. Watching the videos on making the chamber is actually giving me a bit of a chuckle. I see people building these really complex steel chambers with hundreds of bolts as if they were expecting 50k PSI. I bet I could do this with plexiglass or even plastic. I can even add several pressure relief valves that open just above the threshold.

"Avoiding explosions is rather simple." – now, why does that read like famous last words?

I promise I will capture it on video so you can laugh at me.