there are several reasons NOT to build large flat prints directly on the built-plate; the best is the large force created on the print as the plate moves up from the tank that can pull the print or print details off the plate and print; Also you can laser burn damage the surface of the tank from repeated laser hits in the same area that reduces its ability to release the print.
Even having a tank with a glass base instead of a coated base there are still some of these issues. Glass tank bases unlike coated bases rely on the glass to have a minimum hold on the print. But glass can be damaged by repeated laser hits in the same place as well. However, for small areas it may work. I'd be careful not to damage your tank.
IF YOU ARE GOING TO DO IT ANYWAY: When printing directly on the build platform, choose a flat surface of the model to function as a base during printing. Ensure that the chosen flat region of the model has a surface area sufficient to resist the forces experienced by the part during the printer’s peel process. Experiment with different flat surfaces of each model to determine the best area to function as a base.
Building on the Platform works best for model geometries with at least one flat surface that can face directly parallel to the build platform. Shapes without any flat areas, such as spheres or organic shapes, are not recommended for Plate printing these geometries will print with slightly flattened base layers due to early layer compression features on some printers. Also add a lift-off notch to the base of your print as a aid to lifting the print off the plate.
I think to just tilt the print slightly avoids any of these issues. And I use a different type of laser printer to cut and engrave flats.
