This uses 4" dryer ducting hose to connect a fan to a nozzle for blowing air under the covers. 

I tried this with a box fan, and also a squirrel cage blower fan. The box fan in this design works poorly. The pressure in front of the fan becomes too large (as in: you try to compress it too much), it's easier for the air to flow out of the backside of the fan. I do not recommend using this with a box fan, or any fan designed for high volume low pressure. Here's a great Reddit thread about why the low pressure/high volume fans are not good for this type of thing. This design does work great with blower fans or centrifugal fans. Basically fans designed to push air through small holes work well. I stopped using this because it was too cold, and I wasn't able to reduce the speed enough on my blower fan to make this comfortable.

There are several commercially available products that do something similar, but do it better. The bedjet blows ambient or heated air, or the mattress cooler air only blows ambient air. This print is a cheap way to try out that cooling method. We get maybe 2 hot weeks a year here in the Pacific Northwest - so it's hard to justify that kind of expense. At the time of this posting, the dryer pipe cost me about $8USD, plus the cost of filament.

I used a 4" dryer duct because it's the most common size in North America. You should be able to find the foil dryer ducts at the local hardware store. I've found them to be about 30% cheaper online, plus online I found the vinyl wrapped hoses which look a bit nicer.

I used a trash bag and tape to connect the pipe to different fans. I'm sure there's a nicer solution out there, but this is quick and cheap.

The dryer pipe usually comes with two pipe clamps. Use one hose clamp to connect the corner of a trash bag to the pipe. (The ‘hose_to_bag’ piece). Wait until after connecting the corner of the bag and the pipe clamp is tightened down before cutting a hole in the corner. Use the other hose clamp to connect either a ‘neck_extender’, or directly to the nozzle.

I use three sections of “neck pipe” for a king size mattress, ymmv.

There are holes in the side of the neck and nozzle so they can be joined using M3 5mm screws. You can ignore those unless your having trouble keeping things together. Friction fit should hold them in place, or you run a strip of tape around the pipe intersections.

The retention clip slides under the mattress to keep things in place. A rubber band keeps the nozzle firmly in the clip. A sheet on top of the nozzle also helps keep it where it ought to be.

In retrospect, the nozzle and neck don't need to be round. It would have been more efficient to convert directly from a round shape to an oval shape, allowing for more neck pieces to fit on the print bed, and less shifting under the sheet.

All files are designed to print without supports at .3 layer height.

I've attached a parameterized source file if you'd like to modify.

UPDATE: User @rysaw had the excellent idea to attach a 4" inline duct fan directly to the duct tube.

UPDATE 2: User @RobertEaglen_2941542 found a similar design over on makerworld, the “bed rocket”,  which has a base for a 4" inline duct fan. The fan base could be combined with this print to get rid of the ducting tube entirely. Neat!