Hi, I agree, "Spitties" are great! Another ingenious JB design.
I have looked for custom stock options as well.
As with many other custom things that I have tried, I ended up concluding that the original stock is the best option (for various reasons, but mostly originality and simplicity).
Stalker Rifles has made a custom Spitfire stock. See photo from the John Bowkett fanpage on Facebook.
I made a stock for my Firebird (90% the same gun) about a year ago. It was a hell of a job. Quite complicated inletting to put it mildly.
Didn't turn out too well imo. It was comfortable, but "comsetically unappealing".
I found it especially challenging to get the front stock screw right, at exactly the correct angle and place.
No custom stock people wanted to help me. Maybe the man in Poland? I did not ask him. He will probably be very helpful.
I've also heard that the Theoben Fenman stock can be made to fit the Spitfire action.
Theoben Eliminator for Firebird (the Firebird action is a big longer).
Hope this helps.
Cheers