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You can use the likes of a culvert rubber o- ring on the reel to stop over run and hold a little weight on the reel. Leave it on until hooks are on then let it go.

I'm thinking since battery ones they might be hard to find.
They were a cylinder shape about 1.5 mrs through and 3mtrs long.
If you can find a plastic bag like this your away.

No photo. It's just a big bright coloured plastic bag fairly thick tie a knot in one end. Hold in the wind to fill it with air. Then tie a knot in the other end and attach your rope at each end, like...

I take it you know what a kontiki balloon is. I havnt brought one for a while but I'm sure there cheap too. Maybe you just like making stuff, which is cool.