I thought i would add a photo of the fiberglass bass box that will fit in under the floor in the boot. I have done a trial fit and it sits slightly proud on one side due to the carpet so i just need to trim this slightly. This is a push fit and once in does not go anywhere. I bought this from a chap advertising on ebay. He is currently building his web site so only has a facebook and ebay page.
He says he can make anything and has been very helpful.
I was trying to find someone that sold the BSM kit in the UK and stumbled across this so thought i would try it out. It cost me £109.99 including delivery, carpet trimmed, lined with wadding and gold plated terminals. It's a sealed box for a 10" and will fit E90/91/92 with the standard trim piece in situation. It won't fit in a 335i/d