Translation:Was this movie scary?
It is past tense. the movie WAS scary こわかったです。 the movie IS scary こわいです。
So-called い-adjectives kind of include the auxiliary verb "to be" already and can be conjugated by themselves. E.g.「こわい」= "It is scary." The added です in case of neutral Japanese is for added politeness only. So when you change it to past tense, only the adjective itself changes: こわかった (= it was scary).
こわいでした or こわかったでした would both be incorrect.
having 'はこ' + 'わかっ' + 'た' is needlessly confusing to someone using the word box selector. No one actually typing in Japanese is going to mentally form the words that way.
"Was this movie scary" sounds very unnatural in English. Then again it sounds a bit odd in Japanese too.
Did the person fall assleep while watching it?