I've just racked this for the first time. I followed my original plan and added the elderberries to 2l of Lidl rgj and 875g of sugar. Its's had a peculiar floating mat of gak all through fermentation which has never sunk despite regular shaking and swirling but other than that it has been...
On the bright side, at least it's not below the level of the beer. Both your ideas might work my only worry would be that car filler, being an epoxy resin product might weaken the barrel and cause more problems. If you just need a quick fix while you empty the barrel why not just slap some...