I originally had the Stix floats but after 40m they shrink a lot like chewing gum. Even at 30m the loss of buoyancy is small but noticeable.
Here in Italy I bought a pane of this:
Then I cut it in pieces exactly like Stix cubes. I've been using them since two years without problem even at 100m.
Now they produce these rings already cut at the right measure and 200m guaranteed:
I have a small piece of this:
It's wonderful but very expensive. Actually closed cell high density PVC or closed cell high density PVC or polyurethane foam is very common but minimum quantity sold is a 2 square meter pane... Maybe in some small shipyard you could have some scrap. Closed cell foam is used for filling boat decks. (be careful: at least with a density of 75 kg/m3 which bear at 13Bar).
Or maybe on your side you can find some other vendor.
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