As mentioned in this log, I need to go out and start looking for and designing shorter motors. This log describes what candidates I've found so far.

I'm now getting under 1W of power for all shortlisted motors. If these numbers are actually accurate, it means that a solar powered Tetrescent would make sense:

DIY Toroid

As you can see, the difference between one pole pair and two is the difference between being the worst or best in the lineup. I have yet to see if reducing the wire diameter to 0.15mm would be worthwhile.

There's also this graph from the paper:

What I understand is that the flat part of the curve is current limited for continuous operation, and I believe that this relates to power dissipation. If I extrapolate and calculate the 0RPM wattage, I get 712mW. Considering that this DIY motor has a lower diameter and higher length, I'm assuming that it would have a better surface area : volume ratio and so its power dissipation would be at least on par with the one created by the authors. Hence, it should be possible that all these motors should be able to produce 300gf on the belt continuously.

XBD-1636

It's another Sinbad Motors offering that has a worm gear on it. I might be able to ask them to remove it, but it's not like I'm in a rush due to the listing saying it's £28/ea (and the actual price is likely higher). 

TT-1640-24V

So I once again asked TT motors for information regarding a 1636 motor, and I got a datasheet for a different motor lineup. Maybe it's the same thing just ever so slightly longer:

I don't know why they didn't put this handy page in any of their listings. The 24V model was easily the best option, and from calculations, I'd only need a quarter of a watt to get the max desired force output.

It's not all great though. It's $22 per motor.

What about brushed motors?

I was curious to see, since there are more brushed motors in this size range. Well, most of them have a gearing on them (17 : 1 and up) and power requirements that were too high. Even the best option I found, which didn't have gearing and had a handy double-shaft so that I could attach a magnet to the other side, used over 2.5W:

It actually took some digging to find the datasheet for this motor. I actually just found a typical AliExpress listing that just had cool images, like this one of it's skewed rotor: