the same has happened to me, and I made a stronger joint out of two off-the-shelf steel profiles
(one to join two wooden beams and a flat plate).
I used a 10mm steel shaft as hinge, also supporting the lower part of the Mantis head with another bracket.
This is necessary, for my Mantis is a lot heavier than normal, as I mounted big lenses onto it,
for those interested, I attached pictures of my modified mantis head as well.
For normal viewing two stacked achromatic macro lenses (62mm Marumi DHG Achromat Macro 200
plus a 62mm Nikon 6T achromatic macro lens) with some 60mm aperture,
and as my own SLWD solution a Sony VCL-HGD1658 teleconverter plus the main lens
of a high power Rodenstock X-ray objective (I think it was a 90mm F1.0) as macro objective.
Those bigger lenses are mounted on a rotating disc with 3 pcs. 62mm filter thread adapters glued into it.
I replaced the mirrors in the mantis head by bigger ones, and the fixed lens in the head also by a 62mm Nikon 6T achromatic lens.
I lined the inside of the Mantis head with black paper to reduce internal scattering, and added a sunlight shade
because sunlight on my forehead did reduce my viewing contrast too much.
The exit pupil of this modified mantis is almost twice the diameter of a normal Mantis,
thus gaining a lot of freedom while working on a PCB (see picture).
The optical quality is superb, and almost no distorsion is visible.
My main working lens has a 36mm FOV with 115mm freedom above the PCB with a 60mm exit pupil,
and my extra magnification lens has 20mm FOV with 90mm freedom above PCB with a 36mm exit pupil.
I can give more information for whoever would like to upgrade theis mantis as well...