How low would this 6'' x 3/4'' opening be? Also, would this be in addition to more room left open above the maintenance door? By itself, I think that opening would be too small. The drainage tray doesn't need to reach too high up as it only collects the water on the floor already--or at least it should. How are you getting the water out of your drainage tray?
Here is a pic of mine down below. I used a washer machine drainage pan, and then cut corrugated plastic to size to make it look a little neater. I used white tape to fasten down the corrugated plastic. I then put shims under three of the four sides to direct the water down to one corner where I drilled a few holes. Under where the holes are, I put a funnel and tubing to catch and redirect the water into a 5 gal bucket. Also, since taking this picture, I have vacuumed the dirty floor and taken out the trash that is shown in the pic

. My drainage tray doesn't reach too high up over the maintenance door so I can effectively use the chimney effect.
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In this pic I don't have the front covered with window shrink film, but you get the idea. Is there any way to reduce the height of the tray, even just in the front?