Not sure if you're asking how to drain the buckets or how to drain the runoff.
I doubt it's how to drain the buckets because all you need to do is drill holes in them.
So, it must be the runoff.
I use a kiddie swimming pool. It fills my entire room and I have it stapled to the walls. Then, I line that with a plastic tarp that goes a few inches up all 4 sides.
Most runoff is caught in the tarp and evaporates quickly. Once or twice I have had to get some paper towels and soak up the runoff because I didn't want to extra humidity. There's never enough runoff to get to the pool.
I am also going to elevate my pots this time by setting them on 1" rigid PVC pipe. I like to see what's draining out of the bottom.
LOL--I never grow bucket to bucket. IMO, that is far too crowded. I like my plants to have a little room around them. However, even if I put a shower pan with a drain in my closet, I would still use individual drip trays--different strains take different strengths of nutes and some are thirstier than others. We each have what works best for us.