Hi,
afaik many spotting scopes have a filter thread for standard photo filters - either in the dewcap as in this case or - more commonly, in the objective lens mount just before the front element. The latter position might be a bit awkward for installation - depending on how far the front element is recessed inside the retracted dewcap - on the other hand then the decap can protect the - possibly fairly expensive - filter a bit...
I'd probably look for a high quality UV filter in 95mm - the usual suspects like B+W, Hoya or Heliopan will not be quite as expensive as the Kowa accessory.
I personally don't have a choice as the TSN-3 (my usual scope) has no filter thread and also sports a quite sturdy metal dewcap which probably protects the front element better than a filter (except from a stone directly from the front ;-)
I would consider using a filter when I was habitually using the scope in conditions which might be detrimental to the front element coatings - like salt water spray or a sandstorm.
Joachim