I made myself a set of three perspex washboards to fit the space normally occupied by the 'door' to the companionway.
Firstly I got a sheet of 10mm Perspex ( tinted acrylic sheet) which comes with paper on both sides making it easy to see cutting lines,slightly larger than the original door.
Then using that door as a template mark out the outer shape.
Then I divided the height of the door by three and ruled two horizontal lines across the new door.
Using a standard circular saw bench fitted with a high tooth ratio blade ( usually sold as suitable for cutting aluminium), I carefully guided the sheet into the blade; it cuts cleanly and was of course a 90 deg cut.
Then using scraps from the original sheet I cut overlapping battens about 50mm wide, making sure to make them sufficently shorter than each of the cuts, to be able to fit in the vertical sliders. Then using the German made Acrifix glue for perspex attached the battens to the horizontal joins on the inside upper edge of each wasboard panel.
It works a treat and I can have one two or three in place depending on sea state.