Re: Barn door made from MDF and installed onto custom bulk head
Welcome to the Bunnings Workshop community @TomG. It's brilliant that you've joined us, and many thanks for jumping into the conversation.
Hopefully, @LePallet can get back to us with some details on what thickness MDF sheets were used. I suspect that since 18mm MDF was used as it's the thickest available off the shelf. Or, potentially, a combination of other sizes. Two boards would be needed to achieve 36mm, and one of those sheets cut out to suit the pattern. Another 3mm sheet could be added to get you close to the 40mm mark. It looks like @LePallet has cut the pieces for the pattern out individually. I wonder if you could mark out your pattern on the sheet and cut just the voids out with a jigsaw? That way, you wouldn't have to work out all the different angles and use filler at the joins.
A 40mm thick door constructed from MDF would be extremely sturdy.
I'm looking forward to hearing more about your project, and hopefully, @LePallet will get back to us soon.