Rails of correct section available from YRM.
A mate fitted a gas strut on his, says it is nice door can remain open on a side slope or in wind. However when he opens door a bit off it goes flying to a crash as it stop, you have to hang on to it. It is of the type that fits at the bottom instead of the original check.
There are better engineered ones that fit at the top of the door that use a longer slower expanding strut, had one on my Series 3 LWB some years ago and was much nicer to use than mates one.