As-delivered, the wingtips are fairly flimsy and prone to oil-canning. To rectify this I decided to follow the same trail previously blazed by others, adding stiffeners to the insides to make them less flexible.
With the wingtips clecoed in place, I squeezed various spots to identify the weakest areas, and used masking tape to mark some proposed locations for internal stiffeners:
I bought a bushel of 1/4" dowels – which turned out to be more than I needed, but thank goodness for generous return policies:
I marked out where I wanted the dowels to live, cut them to length, and beveled the edges. Then I temporarily attached them to the fiberglass with CA glue, using weights to keep them in place while the glue dried:
Sort of a whalebone corset effect inside the wingtips – I guess in aviation terms these would be stringers:
I mixed up an epoxy/flox mixture, leavened with a bit of micro, and used it to more permanently glue the stringers to the fiberglass. I tried to incorporate a nice fillet as I went:
Then I laid a strip of glass cloth over each stringer:
It doesn't convey in a photo, but the wingtips are now immeasurably stiffer, and demonstrate much greater resistance to flexing:
I'm totally happy with the outcome of this modification – for the cost of a few work sessons and some inexpensive materials, the wingtips are now much less likely to crack due to aerodynamic forces and ground-handling mishaps.