roofing material works. line the inside of a tray with it and use liner adhesive and some sort of sealing compound to join the seams. you have to elevate the tray so you can put a spigot on it and gravity drain it into a bucket or straight outside?
also its important to water slowly, with a watering can
the can disperses the water into a soft rain, reducing splatter on the soil and helps you thoroughly water
I find when you water from a jug and do it quickly the water just finds the quickest way through the soil and keeps going that way, making lots of water escape
an option instead of a watering can is hydroton balls. the balls will make the water disperse more evenly and also ensure that there's no splash up. also keeps the soil down and looks neat
i just use a thin layer, they're reusable too.
so maybe you could get away without making a fancy gravity drain tray setup. I dream of a nice drain setup but isn't going to work in my space
i take everything out at once and water them all slowly, so the moisture travels evenly throughout the medium. once they're moist i put them in a cooler and water two at a time until a little runoff water comes out. then i put them onto an upside-down big ass tub lid, this hold the extra runoff. I proceed to dump the cooler water into a bucket saved for my garden and then water the next two plants . there are 23
after theyre done dripping i take them all off, dump the lid into the bucket and put em back in. The key to having the water not keep coming out is to water slower, and softer. I alternate watering each bucket for about two seconds at first to save time . once the medium is moist it makes it easier to dump water in there quickly.