A 3D printer unlike any other, fits inside a spool box? - Positron V3 intro & Design Story - YouTube It seems to have plans and details.
Oh, this is the upside down printer. I think I saw it 6 months to a year ago. Agreed, it is interesting. I can see how some people could make good use of it at get togethers.
I like it! Hope a kit or design files, released soon.
V4 with transparent aluminum build plate. 
I wonder how it would do with TPU. They only mentioned PLA and PETG in what I watched.
Well, TPU is not that hard.
You should have a direct extrude. Bowden is probably not a deal breaker, but creates more issues with retracts and requires a very narrow extruder outlet (see next point)
Your extruder should also have next to zero space between the extruder gears and the inlet/outlet. If you look from the side onto your extruder gears there should only a minimum amount of filament being visible before and after the gears. You can print little triangular inserts to glue into those areas, if needed to fix the problem. Preferable in ABS/PETG as the extruder motor is getting hot during prints. They can stay installed, when printing other materials and should even make inserting any material much more easy.
It is also useful if your extruder has a tensioning screw. If the gears are set to strong, TPU wants to go even sideways, when the pressure is to strong, while other materials grind. My TronxyX5s extruder spring has a screw (basically the screw head) I need to remove. This results in giving the spring 4-5mm more length and lowers therefor the tension. This extruder tension is probably more problematic than the gabs I mentioned before and after the outlet and inlet
Beside that you just need to print at 50% speed (25-30mm/s), turn off supports and you are good to go.
Here some links:
First a replacement of extruder parts: Ender-3 TPU Extruder Fix by exeswizz - Thingiverse
There is an image showing the filament path and a red line, where the gab is closed. Don´t get confused here. The bottom side is usually covered by the motor gear, when mounted and was removed as the before/after images where taken flat on a table.
Second the insert I talked about above: (these are not the exact the one I used for my GeitPrinter, but similar ones)
You will probably find a matching part for your extruder when searching. Those are available for more or less every printer. Luckily my TronXY X5 has this little triangular guide build in. It could be bigger, but it works as it is.
TPU is a great stuff to play with and there are so many things you can print with it. I created door bumpers, towel holders, hose adapters and so much more.
TPU is such an interesting material. It prints very well on my Ender 3 pro with a Bowden. I can’t believe how strong and elastic it is.