That's awful -- humidity issues are the most difficult.
You may already be doing this, but this may help if you've not tried it yet. Get some aquarium air hose tubing (or an old IV line, or any kind of tubing really) that you can fit onto the end of a large syringe (the 60 cc size work great, and you should be able to get them from most feed stores or veterinary offices). Thread the tubing through the vent hole (or if using a styrofoam incubator, create a new hole just for the tubing). Tie the end of the tubing to a sponge, and you can re-saturate the sponge easily from the outside, without having to open the incubator. If everything's in place, it should take about 10 seconds to do. If you get a gang-valve for the aquarium air tubing from the pet store, you might be able to branch out the tubing inside the aquarium and saturate multiple sponges at once. You'll just need to do a few test runs to know how to adjust each branch, and approximately how much water to use each time. Evaporation from the sponge, which is what creates the humidity, is determined by the surface area of the sponge, not the volume of the sponge, so multiple small sponges are much better than one large sponge. Also, different sponge qualities can be a factor. Some sponges don't fill with water as well as others, or don't distribute water throughout the sponge as well, or are so fibrous inside that they trap water in the center and not the surface, etc. There's lots of variables with sponges, so try different ones.
Also, there's a lot of other things that could help without taking up much space. How about flannel? When flannel gets wet it can really hold a lot of water, but has a huge surface area that would provide evaporation. It also absorbs water and distributes it very well. So if you fill the humidity wells on the bottom of the incubator and put a corner of flannel in the water, it will wick the water up into the fabric and increase your evaporative surface.
Or here's something really out of the box. Go to your local veterinarian and ask them to save you an empty IV bag and a used IV line, which they would likely do at no charge, or simply buy the supplies unused, which should be under $30. Empty out the IV fluids from the bag (if not already empty) and rinse well with water (if the line is used, run water through it also). (If you don't know how to disassemble all the parts and get water into the bag, ask them to show you. Different types of IV bags will be different, and some may require you to inject water in through an injection port, which isn't difficult but requires a large syringe and needle, so be sure you have what you need to fill it with water before you leave the office.) Set up your humidity wells, and/or set up your sponges, or whatever it takes to keep the humidity up, then hang the IV bag above the incubator (because it's a gravity drip) hook the IV bag (full of water, and with IV tubing attached) into the system, adjust the drip rate (there's a valve on the IV tubing that allows you to adjust how fast the fluids go through the tube) to whatever it takes to keep the wells filled, the sponges wet, whatever system you've got set up. You can have that liter of water (about 1 quart) go in over 12 hours or 72 hours or anything in between, depending on how much water is needed to keep everything wet. And it all happens without you standing over it!!! It'll probably take a few runs to figure out how fast you need the water to drip in, and it may change with the number of eggs you have in the incubator, or the number of sponges or whatever you have to create evaporative surface area, but it's a really cheap way (potentially free if you get used stuff) to create a humidity drip line. Just be really careful to keep things very clean, and maybe change out the bag and line every 1-2 hatches to avoid introducing infection into the incubator, just like with the sponges, since none of these items can be thoroughly disinfected.
