Aquarium Cave (DIY) - Cleaning Question

I'd been thinking of buying a small cave for the aquarium but hadn't found one to the specification that I had in mind until it suddenly struck me that the dosing top on Persil Silk & Wool http://www.wilko.com/washing-powder+fabric-conditioner/persil-silk-and-wool-liquid-750ml/invt/0321121 actually "fitted the bill" completely ie suitable size, sufficiently see-through to keep an eye that no-one started spending too much time in there, no-one likely to get stuck in it, etc.

I'd just like some reassurance that, if I soaked it in water with dechlorinator for 24hrs before putting it in the tank, that should be sufficient to remove any traces of liquid detergent and that it should be aquarium-safe?  I have Milton too - ought I to use that beforehand, as an added precaution? Thanks.

... I would be inclined to soak the ball for a week, changing the water a couple of times a day.

I used this method to wash out an anti-bacterial hand-gel dispenser which I wanted to use to dose plant foods. Why take the risk?!?! (it is just the perfect size and dispenses 2ml per squirt so it's ideal for the task)

Of course I was afraid that the detergent and the alcohol could harm the fish and the bacteria in the filter, but using a similar method to the one Sue has proposed it was fine.

However, it's not for the faint-hearted, I was soaking and washing out fresh water several times a day and then overdosing with dechlorinator for well over a week.

How did I decide it was safe to use? I filled with tap water and shook vigorously to ensure any residue would dissolve into the water, and then I tasted the water (I didn't swallow!) Only when I was certain I could taste no soap or alcohol was I confident to use it in the tank.

Maybe soak it for a couple of hours in Milton solution, then give it a good rinse. If you are worried about any bleach (= Milton) residue, then a final rinse in water slightly overdosed with dechlorinator.
I tend to just give things a thorough rinse in cold water, and don't bother to dry or use dechlorinator. The amount of chlorine in the water on the object will do no harm to an established tank's bacteria colonies. It's newly cycled tanks that you need to be careful with as their bacteria won't have embedded properly in the biofilm yet.