Looks like to me you are pretty good at welding. Finish up the job and you will be able to add coolant with no issue. As far as a drain this is also a good idea. You would weld a threaded coupling to the bottom if you drill the right size hole. This find a plug that fits the coupling. You can usually find that stuff in the big box stores like Lowes or Home Depot. Is the enclosure aluminum or steel?
Russ