Hi Andy
Try searching under the terms "hatch" or "round hatch" you should give you some better results. Lots of old post including how people have fabricated the round hatch as well as the wood spacer that goes under it.
You can also try searching my web page
http://canadianmilitarypattern.com/ and type in hatch at the search feature at the top of the page for some detail photos on my HUP. Most of them are cross posted here to MLU but scattered over many years and posts.
Posting a photo of what you have and what you are working on will also help people focus on your need and question.
Cheers Phil