You might put a c clamp or vise grips or bolt a wrench to the front of the trans bell to keep the converter from falling out on the ground.
You will only drop it once then you will get the idea.
I had the same thought after I pressure washed the engine compartment, so I strapped a really cheesy bungee cord across there (it was all I could find at the time). But it was all frayed and it broke when I was working on the suspension, so I tossed it across the garage.
Car is up in the air right now, and I doubt the converter is going anywhere, but you make a really good point, and I'll take care of this tonight.