Corner pieces for surround for 3D printer or cnc router. Slots need filler but make boards or plastic sides removable for access.
Suspend over lathe for holding a camera and join the ever growing trend for awful youtube machining videos.
Framework for lightweight loco driving trolley. T-slots hold floor/seat sheet and allow removable footrests and guards.
Slides for underbench tool drawer or sliding tool rack inside lathe cabinet.
Frame over lathe to hold overhead drive motor for topslide held drill, mill, or grinder spindle and lights and coolant pipe.
Mounting for DRO display on mill or lathe.
Mounting for vice end stops and swarf screens on mill.
something similar to French cleats.
Culling lengthways into two L shapes may add some more options.