Thank you! That's sorted me out. Ended up paying a fortune because anything over 3" is super expensive for some reason, but it'll look nice at least. I really need to learn my threads.
Have you got the steel rocker cover or the cast alloy type?
The steel ones only have a small flange so 1" long will be more than long enough. Hard to get really long ones for the alloy cover (4" I think?) so for my alloy cover I bought some 1/4" unc threaded rod and cut it to length then used four stainless 1/4" unc dome nuts. Haven't actually fitted it all yet but have tested the fit and it's all good.
4" rod screws into head to form studs, slip gasket over the studs, slip rocker cover over the studs, fit washers as desired and finish off with the nuts.
I'll be using spring washers and a drop of threadlock just to make sure they don't vibrate loose.
Got the cast alloy one, I looked at doing it in bar but couldn't resist shiny bolts. They arrived today, which was pretty cool. Just a couple of things to sort and the engine can go in the van!