Wait till your bc has crusted and is firm. I’d make the fence from gumpaste, much stronger than plain fondant. Easily done by adding tylose/cmc powder to your fondant. Once your gumpaste fence is nicely dry and hard..you could use piping gel to attach, as stated above. But sometimes that can soften your gumpaste. I prefer melted chocolate. Spread on very thinly while warm, once you place the fence on the cake, chocolate hardens within seconds. That fence won’t budge.
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