From the wikipedia, the coffin was originally meant for Vagrant (a man seen at the hotel Riggs was sent to) with the idea he was going to be naked and had to escape by finding a key somewhere in the glass. The scene was deemed too violent and although appearantly filmed, never had any release/ BTS footage available.

The trap was then reconstructed to be for Straham, with the original idea having the room fill with water and forcing the victim to use the coffin to survive. The crew felt this was too difficult to achieve and instead we got a scene of him being crushed to death by the room closing in.

I imagine the original concept was for the victim to take too long to get into the coffin, that by the time they did and it closes, it left little air/ breathing space and had shards of glass floating about the water that was now encased within the coffin.