I seem to remember someone having troubles with the cap staying in place while the silicone dried (was that you, Cindy?)
Some tape should hold it on well enough until things set-up & dry.
And, while you are doing the one, ya might as well do the other one too. No doubt it will fall off sooner or later as well.
But don't stop there, as long as the silicone is out, put a couple dabs on the nut that holds the toolbox lock on, it's known to come loose and ya end up losing the lock, which, if you go to the dealer to get a replacement, you're looking at big $$'s for a little part.
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."