Hi, I've been lurking on the forum for a while and can at last add something

My cars a Sterling Grey, 2005 2.5, standard for now, had it since March.
I've just bought a Cabrioshield roof cover from Fleabay this week. £89 plus delivery, came in 2 days. They do 3 colours and the material does feel pretty good.
BMW Z4 Car Hood Soft Top Cover Half Cover Protection, sold by prestigeautotrim. Its a lightweight woven nylon type material.
I had been looking and there are plenty out there similar for about £50. What won it for me was no straps to rub against the body, there are protrusions that fit into the door gaps and boot and a magnetic surround to stop it flapping all the way round. I bought it to keep the crap off at work when its outside, especially the frost.
I think the material will wipe clean, since you mention tree sap. I only got it yesterday and have had a quick trial fit, seems fine. If I get a chance and work out how to do a pick Ill post it.
http://www.ebay.co.uk/itm/BMW-Z4-Car-Hood-Soft-Top-Cover-Half-Cover-Protection-/121164629078?pt=UK_Car_Exterior_Accessories&hash=item1c35f98456