I use a big wide open pan to catch coolant, think I bought at the local parts store. Auto Zone or O'Riley's will carry them.
A hose into a jug is a good idea and should work fine.
might be able to buy extra room, if you pull the splash guard, but not sure, I haven't been "in there" in a year...
The o-ring has a groove around the inside surface and it fits around the flange of the t-stat.
If that's what you are doing, and it still doesn't fit, are you sure you have the correct part number?