certain racing regulations stipulate that you must run at least 1000ml for a catchcan - the obvious reason for doing so is that so it hopefully won't overflow for the duration of the race.
I know on the original setup that I saw on the Spoon cars, the drain for the can ran to the back the car and into a sponge. That sponge would get saturated and would slowly drip during the duration of the race. Personally I don't think this is a good alternative, but I can see the validity of trying to the oil as clean as possible and 1000ml or not, it's probably going to get filled sometime during a 24 hour enduro - unless you run drainback system into the motor.
For baffling, this is why it's important to buy a catchcan that you can open up. This for example is from Megan Racing and would be my choice if I had to modify an "eBay can."
No need to weld inside, just run slash cut tubing from the feeding port to the very bottom of the can and then fill the can with stainless steel scrubbers...NOT stainless wool. Scrubbers will unlikely come apart whereas the wool is so fine, your motor is likely to inhale them IF you run it back to the IM port.
If it's venting to atmosphere, you could fill it with almost anything, as long as it's resistant to oil and won't break apart.