How to keep gapped front teeth closed after using orthofill bands? (Photos)

I have had a gap in between my two front teeth as long as I can remember. I bought orthofill bands from amazon because it was a cheap/safe alternative to braces as my teeth are already straight and healthy. My gap was fairly small, but still noticible. It closed in approximately one month. However it starts to reopen if I leave the orthofill bands off for more than 12 hours. I know I need a retainer but which one do you recommend?

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