Indeed, basotect's absorption falls off sharply below about 400Hz due to its light weight. But yes I have some good sources for it.

What I would suggest is simply rockwool. I have some more of the things I am using if you want to give me a few bob for a pair (£50 each?). These are the prototypes I bought back from my distributor recently and mentioned in my PMC advert.

Here is a picture of one. The fabric needs re-doing but thats no problem.