Screen riser, 98 mm high, accommodates a 249 mm long and 19 mm high board. In the photo, there's a 500 mm long board, and stability isn't an issue. I've added the option to screw the board to the stand with caps. It's my first 3D model, and I'm still learning 3D printing and design, so feel free to ask if you want it remade to different dimensions.

Update 21/05: if you only want caps for screw → https://makerworld.com/en/models/469217#profileId-378550