I have done this before so would like to share some of what I know. Super critical CO2 has some unique properties, for one it is extremely difficult to keep in a closed container since it has near zero surface tension. As such, if a standard threaded fitting is used and the fluid comes into contact with it you can be fairly certain it will leak (and usually badly).
Welding stainless of this thickness properly requires a powerful TIG welder, this in turn calls for a certain level of skill in the operator. If you have both of these then please disregard this comment.
Another option all together is carbon steel. It is inexpensive (compared to your stainless piece), comes in a variety of sizes (DOM is a good place to start), welds nicely with a basic MIG, and is easier to machine.
Regardless sizing and material type are critical, although engineering this piece with an adequate factor of safety will keep things together. Once again this requires a certain level of skill, if you don't have it then this is not a safe project to tackle.
Next you will need correctly rated and calibrated pressure gauges. Installation is straightforward although their importance can not be under estimated. Two is the minimum in case one is not reading properly (stuck needle for example).
Calibrated pressure relief valves are also essential, and once again two is the minimum in case of failure.
If you manage to build a well engineered container then the next step is getting the liquid in it. I suggest first packing it with dry ice (can be found at nearly any supermarket) and then sealing the unit. A bottle of CO2 can be used to maintain pressure although their regulators are intended for low pressure output. If you are unsure of how to perform modifications safely on one of these setups then this project should not be carried out.
Generally what I am getting at is this, a project of this type brings up safety concerns even for a veteran engineer. Playing with extreme pressures and super critical fluids in a home built container is no easy task and could easily result in catastrophic failure.
I certainly can understand the scientific curiosity/challenge aspect(s) of a project like this although there are still other concerns that need addressed beyond what I wrote here. If your skill sets are not in place to at minimum perform the operations listed thus far then I suggest leaving this project alone.
I am not saying you do not have the ability to acquire said skills, just that this is not the type of thing to learn and try them out on.