One of my books (below) has a picture of a very similar looking spider identified as a spiny flag spider, Alpaida cornuta, also from Costa Rica. I wasn’t able to find anything online though using either the common name or the scientific name. The World Spider Catalog doesn’t seem to recognize that name at all, but I saw two species there from Costa Rica, A. bicornuta and A. championi. Perhaps this is one of those.
Here’s a similar looking unidentified Alpaida from Ecuador, so I think the genus is probably correct.
An Essential Guide to Insects and Spiders of North America and the World
by Rod and Ken Preston-Mafham