How do I store pre-made meringues for floating islands dessert?

I'm gonna go a little against the flow here. As it's a poached Meringue, not dried out in the oven, it's perfectly okay to store it in the fridge for a few hours. It's supposed to be soft and moist. They will need to be in an air-tight container (to keep out any strong aromas from the fridge) and on kitchen paper, to absorb excess moisture from the poaching liquid. The downside to this is that they can deflate a little over time, so I'd be looking to cook them to order.

Whatever you do, do NOT put them in the fridge. They will turn spongy and be unusable.

Room temp, definitely. Refrigerators are humid environments and will soften the meringue for sure.

Agree with HLA about air-tight & duration. You didn't sat where or temp. I was taught room temp. Agree, other?

I recall a nice restaurant in Paris that displayed the floating islands on a platter of creme in the window - and served from there.
Keeping them air tight is right, but let them come to room temp and "float" before serving.

Make sure the meringues are dry through and through, then store in an airtight container. Any moisture will make them soften (be it from their own interior or the atmosphere). They will typically hold for 2-3 days.