"Here is the box I found."
Translation:Voilà la boîte que j'ai retrouvée.
Grammatically "voilà" is used for things or people further away or revisit a topic previously discussed and "voici" for things or people close to the speaker or to announce something that is coming. However, in practice they are used somewhat interchangeably with "voilà" being far more popular in everyday discourse. Voici is also correct and added. Thanks!
Here is a link describing the slight differences between the two terms: http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=3862
Chrissielil is correct. Here the direct object (la boîte) precedes the conjugated verb. In this situation, you must make agreements with number and gender between the direct object and the verb.
For example, if I found multiple boxes: Voici les boîtes que j'ai retrouvées. This is also true if the direct object is a pronoun in front of the verb. J'ai mangé une orange → je l'ai mangée.