Depends what you're trying to hold down. If it's an ESC/receiver then a bit of movement won't hurt, it actually helps to cushion the electrics from shocks and vibrations.
If it's anything that's under load, like a servo, then I'd glue it down with Shoe Goo. Zero movement, mega strong, takes a little while to set so you can reposition things, and comes off without leaving a mark when you want to remove it.
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