It will extend to the very edges of the camper if that is what you prefer. On mine, I cut about 6-8 inches off so it was just inside the bottom edge but still out past the frame. That gave me room to put on a cap and, in my opinion, just looks better.
I think, if it were me, I would construct the whole thing but not cut it to length until you get the camper and can actually test fit it. The only end that would need any cutting would be the open end where the removable end cap would go.
I actually made three of them. One for me and one for each of my sons trailers.
Barney
Here you can see it installed in front of my old carrier which would not fit the new hose fittings.
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine