The Fatboy tank is supplied with an 12" screened inlet strainer [and the screws to install it]. There are three positions on the top of the tank to install the strainer, with an indicator ring in each position to guide you.
Cut a hole in the required position using a jigsaw or keyhole saw, and screw the strainer in place. Direct the downpipe from the roof into the strainer.
The strainer is mosquito proof. Keep it free of leaves, and remove occasionally for cleaning.