Mel, I did the LED rope light retrofit in a 4' x 4' double sided fixture about 8 years ago. But used the warm white bulbs that I was never really happy with. So this past fall I took out the rope lights and installed (4) 2' tube light fixtures as shown in the pics...While it works great and really like the new Daylight tubes, its not perfect. You can see some very light shadows inside the sign if you look hard enough. I can see them, but I doubt anyone else can! To fix it, I was able to find 3' Daylight LED bulbs that I am going to install this spring.
What I would do if I were you, is put (4) 8' tube single bulb fixtures inside the sign. 2 top and 2 bottom and let them over lap in the middle. Or you might be able to put (2) 6' in the top and (2) 6' in the bottom. I think you'll be surprised at how well it lights up.
The pics below of my sign with (4) 2' bulbs is only 32w total! 8w each bulb...I think putting 3' bulbs in will be the ticket. Total will be 48w....My new approach will do away with the fixtures and will use c-clips to hold the bulbs. Then just use wire connectors to wire up the pins. Already tested, works great.
You could get creative with different lengths to make up the 15'. Spaces between the bulbs won't hurt a thing as you can see with what I came up with 1st try. You won't have $100 in the whole retrofit. You can get 4' LED tubes for about $5-6 apiece.
