I have a push on fill valve cover on my mk1 Logun,it’s held on with a nice snug o ring.
I wouldn’t know where you could get one from.
There is a bbs member on here The Logunator who may be able to help you.
Les..
Edit
Hi Jason
I have just looked on the Chamber’s airgun spares site,and have noticed the mk2 Logun rifle as sliding dust cover part number CO206.
It’s been a long while since I have had a play with my mk1 rifle.