Cannot control humidity in incubator

[PDXJULES]

Apologies if this has already been asked and answered but I am new to here.
I have a small, simple incubator (polystyrene box with light, fan and vent holes.
The way I am humdifying it is via a ramekin filled with warm water which gets the humidity to 60-65% with some vents taped over but it isn't constant and when it comes to hatching I cannot increase the surface area or humidity as I have to open up the vents for more ventilation.
I don't really want to spend out on a humidity controller at £70.

Any ideas??? Can you make a homemade humidity controller???

Thanks

A full tray or wet sponge would probably spike tbe humidity too high except at lock down. But test it first, and recognize there will be fluctuation with embryo growth, requiring a little squirt of water in side vents ( not on eggs) several times a day if you want humidity at 35-50 the first 17 or 18 days. A drop at pipping, and fast rise may happen at a hatch...so those are clues. You can also add a small amount at turning and candling...probably not more than a teaspoon. Be sure a hygrometer is inside you can see without opening the lid. Good idea to read about dry hatching and know the usual humidity of the room the incubator is in. Many people have had successful hatches in these. The shape allows frequent tilting in between gentle hand turning. I used that for a decent hatch in a similar one ...moved 9 chicks to the brooder today. They were Shipped eggs gathered an transported ain freezing temps, so i am happy at an over 50% hatch rate!