When solder paste printing a PCB with a stencil you want to hold the PCB and stencil securely. These brackets are typically at the exact thickness of the PCB so they hold the PCB securely and keep the stencil flat. To generate your own customizable brackets use the Open In Customizer button on this page and enter your custom dimensions. These brackets are similar to the ones sold by oshstencils.com and others but 3D printing them saves you the waiting time and give you more size and thickness choices.