Another possible way to stiffen the arms, without any need to modify the original structure, would take advantage of the unique mechanical properties of Ultra High Molecular Weight Polyethylene (or UHMW-PE for short). It has exceptional mechanical strength to impact and stretching, even in relatively thin films and filaments (UHMW fibers are used to make Spectra and Dyneema rope).
UHMW tape is available on Amazon in various widths and thicknesses. It's a milky clear, so blends in reasonably well. A thickness of .005" or .006" would be good in this application (thicker starts getting too stiff to make nice right angle bends in the tape). Apply the tape lengthwise on the bottom of the channel, squeezing the channel sides together slightly and folding the edges of the tape up and onto the sides of the arm. The acrylic adhesive should adhere well, but is removable if needed prior to warranty repair etc. The UHMW should work in this application, as it would behave much like the diagonal drag wires in a fabric covered airplane wing, which give fore and aft rigidity to what would otherwise be a relatively flimsy bunch of rectangles formed by ribs and spars etc.
Left over tape is useful for all manner of anti-chafe uses, surfaces that rub, etc. as the material is more abrasion resistant than most steels, yet almost as slippery as Teflon.
