You don't say how big a project
you are doing or where the pieces will be (ie. above the waterline or below). It's hard to say for sure, but if the pieces will be in an area that is normally dry I would go to Home Depot or such and pick through their pile of 2x4's for some clear stock with straight grain. It's likely to be spruce, fir or douglas fir depending on where you live. All will take a fastener and if you coat the pieces with epoxy or paint
them they will hold up very well. The 2x4's are cheap
and it's easy to hold shorter pieces against the hull
to scribe to fit. If you are doing a major job you will probably want to got to a specialty yard to get more consistant quality than picking through the pile. Black locust is pretty hard stuff but it will probably outlast the fiberglass! Good luck!!