How big are your box joint fingers (er, hope that term doesn't spark another argument)? Given that the LS allows repeatable positioning, multiple passes at increasing depths will probably be ok though you might not wind up with super tight fingers. Since the board isn't that wide, you can probably get excellent contact just by clamping across the fingers.
By the way, you aren't limited to your router bit width as the finger size. Here's an example template that uses a 1/2" bit to make 5/8" fingers. Note that it shows where you make multiple passes to get the right width finger/slot. If you have an accurate printer, you can cut that strip out, reinforce it with clear tape and slide it into your LS.
Measure twice, cut once and CROSS OUT THE WRONG MARKS.