We store a caravan in Cornwall HERE
... I know it's not Wales
and their rates are quoted for caravans
but there are boats in there
It's certified 'secure' to an insurance company recognised standard...and their prices are based on length, axle count and duration of stay. All the storage is covered. Access to caravan/boat/trailer is by appointment only, and, they deliver if required.
I suspect they're at the top end of the scale but might give you a bookend?