Earlier Mass Portals used two rubber O'rings per delta side to pre-load the ball joints (one next to each joint pair). These perish and fall off. The last unbroken O'ring of the six that were fitted, sits in pride of place right in the centre of the image. :)
This is a single TPU belt that replaces both. While possibly not the best solution, it does work.
Tension can be adjusted for your printer by scaling down or up along the Z axis.
This belt was made specifically for the XD20, so I don't know if it will fit other Mass Portals as it is.
However, with a bit of crafty scaling it could be made to fit just about any delta printer - not just the Mass Portals.
Layers: 0.2mm, 2 top, 2 bottom, 2 shells
Infill: 100% linear
Print Temperature: 220°C
Bed Temperature: 70°C
Filament used was rigid.ink TPU (Colour: Indigo/Purple)