We use ECCO self-contained strobes on aerial ladders for firetrucks where I
work, and they seem to work well and I believe they are a lot cheaper than
Whelen. Check out their web page, perhaps you can find something that might
be useable on your airplane. In particular check out their 9001 series,
those look like they might be adaptable to an aircraft wingtip. They aren't
self contained though, you'd need an external strobe pack (power supply).
I don't remember the model number since I left the aerial ladder
department almost five years ago, but we use some strobes that are
completely self-contained, all you had to hook up was 12VDC and ground,
the power supply is built right into the mounting base. Those lights were
maybe 3 inches in diameter at the mounting base. I believe you could
probably figure out a way to put those in your wingtips too. Let me know
if those sound like something you'd be interested in and I'll try to find
out the model number and price when I go back to work Monday.

http://www.eccolink.com/ProductPages/Products.cfm

Scott Wilson