I have the same problem!
Keep your eye open for pressure washing companies.
I came across a couple of lads pressure washing some paving about three months ago - they had some stuff in drums, a silicone substance, I think.
Their drums had the same tops as mine, and since they were chucking the drums away, I managed to scrounge 2 tops.
Wish I'd got their address..........I'm missing another top!
Baldeagle