My forestock wood flares along with the barrel, so the cap must conform, too. That doesn't make it right, it's just what I like to do.
It's been 15 years since I last used a cast cap, but I do remember struggling with it. But I struggle with every cap, each has its own personality.
The cap I used was smaller at the muzzle end, and it flared larger toward the open end toward the lock. I recall I fit it to the end of the stock, and did a tremendous amount of filing to get it to be straight-ish with the top and bottom lines. Hopefully, your cap will be smaller than the barrel, so you will have to file the inside of the cap to fit the contour of your barrel. Save this fitting until AFTER you've fit the cap to the wood. You may have to squeeze the cap smaller to be snug to the barrel sides.