This is a pretty simple DIY modification to a 5 gallon bucket with lid, and a $15 commonly available humidifier. The purpose of the modification is to harness the heat generated in the water reservoir by the steam humidifier powerhead, even after it has "run out of steam". The original unit is quite effective at raising the bedroom temperature at night, without much noise or electricity usage. It also provides steam which can help with sinus problems or other ailments. The modified unit has a 4 gallon larger water reservoir with a lot of surface area, and can create even more room heating capacity. It does not run longer (by much if any) than the original, before needing to be refilled (remove the power head and fill as in normal use). It does retain heat longer, though. This modification also provides a welcome carry handle.
Step 1: #1 Warnings and Tips
Do not attempt to suspend or hang this device by the carry handle! (it could fall or twist up the power cord or spill).
Be careful to have proper ventilation and protective equipment when handling soldering irons or razor/hobby knives. Don't try to use kitchen knives. It will probably break, or break the plastic, or cut you.
Follow the original operating instructions, including how much salt to add to the water. If you lose the directions, when starting the first time, add 1 tablespoon of plain salt to the 4-5 gallons of water and stir. Do not add more salt unless the device stops producing steam even when full and warmed up.
This modified tank will be heavy (about 40 pounds full), so best to place it on a piece of plywood, flat paving stones, a floor mat, bath mat, or carpet remnant, to prevent dents or damage to carpet/flooring.
Do not sleep directly next to the warmer. If it sloshes it might spill hot water on you. If too much salt is added, it might shoot out squirts or drops of boiling water. Normally it just makes a little steam all night and then sits dormant until refilled. Unplug when not in use!
Step 2: #2 Modifying the Bucket Lid
This is not difficult, but is time consuming, and requires careful work.
Edited: I have found the quickest way to make these is to use a Walmart tomato juice can as a template. Trace with a fine-tip Sharpie, and use the soldering iron to trace the line. Use the hobby knife on the groove, or go around a few times till the soldering point cuts through. A white lid will make it much easier. If the juice can just barely fits through, the humidifier unit will slide right in. You can open the bucket lid or remove the power head to refill.
If you want to use a hobby knife ("exacto" to trim out the center hole, be careful to do it over a safe work surface. Go slow and "score" the cutting area multiple times, Be sure to always cut away from your hands, wrists, and body. When you cut through the plastic, don't get excited. Keep going slowly and etching out the cut. You can use rough sandpaper or half-round files to expand the hole.
If you are using a soldering iron, you should first carefully engrave the cutting area, in a very well ventilated area (outside!) over some foil and over a non flammable surface (like concrete). Basic soldering irons are about $4 at WM dept. stores, in the Automotive section, or find an old one being discarded from an electronics shop. It takes some practice to get the best result the first time. I recommend pulling the hot iron while tilting the tip away from the direction of travel. You should aim for cutting the pattern out smoothly up to the full width of the drawn marker line. Cutting too small a hole (even a fraction of an inch difference in diameter) will make the power head fit too tight, and will require a lot of filing or burning with the iron to expand the hole. Heating the whole lid with a hair dryer to a comfortably warm temperature, will help get started and make the lid pliable.
After test fitting the steam unit for a close fit, but not overly tight fit, mark and make a notch for the small plastic locking tab on the steam unit. It will work if the tab is cut or broken off, but the tab provides a safety measure by making sure it stays in the water reservoir.
After your lid is complete, add the lid to the empty bucket. I made mine from white buckets for better looks. There are colored buckets, and even camouflage buckets available at L's or HD hardware stores, for about $5 including a snap-on lid. The lid will probably just stay on all or most of the time, so the basic lid is fine.
Step 3: #3 Modifying the Steam Unit
The steam unit is a brilliant and inexpensive vaporizer made by Vicks, and marketed widely at pharmacies and general stores for about $15. Save the original reservoir in case you need more portability, or don't want as much heat.
There is a small hole in the bottom of the steam unit, which is where the water enters. It also is where flakes of metal sediments get clogged if you have "hard" water (lot of minerals) or use too much salt. There are two metal electrodes about 5 inches long inside the cavity. Never touch them at all, especially when plugged in. You can prevent clogs by adding two cuts as shown in the photos. This allows the flakes to fall out more easily, and extends the service life. Even with the normal water reservoir, you may notice black or gray flakes in the bottom after extended operation. This is normal, but can be prevented by using distilled water and minimal salt. (Salt is used to create electrolysis, which steams the water.) In the large 5 gallon bucket, the flakes will fall to the bottom into water that can't be drawn up by the steam unit, so they are less of an issue. You can rinse the bucket out to remove them, after the device is unplugged and cooled down, as periodic maintenance, or when emptying it to move or store it.
Step 4: #4 Installing and Using the Room Heater/humidifier
Put down a rubber floor mat, carpet remnant, automotive floor mat, bath mat, or similar, on top of carpet or wood floors, to prevent damage (from condensed water, spills, or the weight of the warmer compacting the floor).
Place the warmer away from direct sunlight, out of the way of foot traffic, and where the steam will not damage cabinets, clothing, furniture, or light fixtures.
If you have pets, keep them away as they might knock it over or urinate on it.
If you have toddlers, or irresponsible children, do not use this device.
It helps to have a few 1 gallon water jugs in circulation, to fill up when you're waiting for hot shower water, and to carry to the warmer to refill it.
If you close the door all the way and run this all night, the humidity may become quite damp, and can cause doors to swell or shift, preventing door and door jam alignment. A door stop might be handy.
Good luck with your room warmer. It can help with apartments or campers, or for a room you frequent a lot.
Added: photo of room warmer with camo bucket. These are about $4 or $5 at major dept. stores and hardware stores. There are other color buckets available, but this is my favorite.