You could try a metal fabricators, and get a lid made from aluminium checker plate, but to be fair if you are looking to get one sorted for less than the cost of the one in your link, you are probably better of doing a DIY job, using marine ply or similar.
For some reason OEM ABS lids for trailers, when bought separately have always been pricey.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets