I have gone for the simpler approach and used your second suggestion. Giving all aimed attacks the added attack roll bonus does not feel justified compared to how the bipod works IMHO.
Fitted Stock [Firearm Modification, Version 1.1]
Cost: 20 gc
Description: This custom-made rifle stock has been tailored to precisely fit the posture, arm length and body shape of the wielder in order to provide a more stable stance when aiming. While simpler and faster to use than the normally employed bipod, the additional stability offered by a fitted stock is also not as substantial.
When a character wielding a rifle equipped with a fitted stock forfeits their movement to aim during their turn, they get an additional +1 bonus to their first ranged attack roll that turn.
The attack roll bonus provided by a fitted stock is not cumulative with the bonus provided by a bipod (see IKRPG, p. 272). Unlike a bipod, however, a fitted stock does not require a quick action to set up.
Morgan’s Notes: Remember to have a new fitted stock made when you switch to a different suit of armour, as the stock needs to accommodate that armour as well. This isn’t usually much of an issue, though, as few people have more than one suit of armour in their traveling trunk.
Further comments welcome, as usual.