Engin Cycles knows a thing or two about MTB design and fabrication. Their hardtails have been progressive before that was a thing. The latest product from their Philly shop is a 35mm clamp stem, available in four colors and 32mm, 40mm, and 50mm lengths. They retail for $150.

These 6061 aluminum stems can be upgraded to in-house milled titanium hardware for a $30 upcharge.

Specs:
32 mm 135 g (w/ Steel Bolts) 127 g (w/ Ti Bolts)
40 mm 147 g (w/ Steel Bolts) 139 g (w/ Ti Bolts)
50 mm 181 g (w/ Steel Bolts) 172 g (w/ Ti Bolts)
Stack Height 40 mm
Clamp Width 50 mm
Height Offset 5 mm
Steer Clamp 28.6 mm
Bolt Torque 5 nm MAX

See more at Engine Cycles.