You don't need the chips to drain out.
That is why Runner4404spd suggested using a screen over the holes. If chips get in the holes, they will clog it like you never drilled them.
I never clean my chips out of the tray the same day I have been running coolant. I wait until the next day or a couple days later and do it before I start milling anything. This way most of the chips are relatively dry and you won't be throwing out your coolant. I use a small dust pan and a cheap paint brush for the bulk of stuff. Then I use a wet/dry shop vac.
When I am setting up after having run some parts, I use some small shots of compressed air from a compressor to clear off where I will be mounting the tooling, stock and hold downs. I use a clean shop cloth just before I set the stock down.
I have a pretty large chip tray, so I only need to clean mine maybe once a week. It's not spotless after I do it. It's a mill.