Hi John
the great thing about wood is that it is easy to work with/on. You could do a few things (including replace with longer strips which is probably more hassle than warranted if you have an internal stem). I would probably put on all the hull strips and then shape a wood offcut to approximately fit into the gap lengthways, then sand back for your external stem piece. You could do smaller sections the same way, it will to some extent depend on just how many levels of strips do not quite meet. You could even just use some good old dookie (epoxy with sawdust and filler)but the wood infill will be lighter if you have lots of big gaps. What ever you chose a nice external stem will cover up any less than perfect handiwork.
cheers
David