Having put a load of sound deadening in the boot/behind the seats last weekend - might do a post on that later - I also added little squares of aquarium foam as linked in the OP this morning. They are about 60x60mm; I didn’t want to pack it in there too tight in case it obstructs the lower flap in the drain and/or prevents water freely passing through. So far on a small sample size, it seems to have made an appreciable difference. Coupled with the sound deadening, it’s quite civilised in there with the roof up, now.
Now I’m off for further testing while the weather holds up… 
Def don’t use that foam, way too dense to let water flow through it, look on eBay for Fluval aquarium or similar filter foam.
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