What was your reasoning for mixing/diluting the urethane?
I would definitely recommend applying it to a scrap piece for practice if you haven’t already. I used spar urethane when I made my front door and while it’s easy to apply it would want to pool up on edges. A stock isn’t a square door but I can see it wanting to pool up where the fore end turns into the barrel contour or the butt pad.
Getting a feel for the amount you’re applying will be another thing to think about. It only wants to spread so far - if that makes sense.
And making sure you get everything applied relatively quickly. If it starts to tack up and you apply more over that tacked up urethane it’ll wrinkle. Getting a feel for this timing will be good as well.
The good thing about it is you can get a feel for applying it pretty quick, looks great in a non semi gloss-gloss, is easily sand-able and easy to apply new layers.
I have not used this on a stock before. But what I would want to try is the French polishing method to apply the spar urethane on something small and shaped like a stock.