A fair amount of slack for a TT bike, access hole to the top tube is quite wide, plus the stem (later covered by a 3d printed top cap that gives that aero finish) has enough room around it, but if you leave too much slack, the outer will push against the 3d printed top cap and the result won't be perfect, last time I opted for some zip ties to hold the outer as close as possible to the stem without compromising steering.
Here's a pic I took from the previous build, outers not installed yet but it gives you an idea of where they need to go through (there is another hole on the other side of the stem):
A fair amount of slack for a TT bike, access hole to the top tube is quite wide, plus the stem (later covered by a 3d printed top cap that gives that aero finish) has enough room around it, but if you leave too much slack, the outer will push against the 3d printed top cap and the result won't be perfect, last time I opted for some zip ties to hold the outer as close as possible to the stem without compromising steering.