making a trunk cover

I'd go with 1/2" MDF covered with non backed carpet.   Are you looking for a factory style cover, or something custom?

...agreeing with austincustoms according to your needs.  Mass retail stores....like Wal-Mart sells 3M spray adhesive for around $8ish.  Make a template to the shape of the insert your trunk hole requires.  Take the template to the MDF and cut out the shape.  Test-fit the MDF in the trunk.  Cover the MDF with whatever carpet you may purchase.  Test fit this carpet and cut to shape.  Spray the MDF and the carpet with the adhesive spray and let them tack up for a few minutes and apply them together. 

Also, I provided a simple search on this site for "Trunk"..here you go...

https://www.the12volt.com/installbay/search.asp?SI=TC&KW=trunk&SM=1   ...just to get you on your way to researching....

...I just remebered that there is a 2005 BMW 525 out in front of the garage, went outside to the garage and looked.  Seems as if your wanting something simple to start out with that maybe you should look into "Masonite" board instead of MDF.  This, Masonite, exhibits an actual thickness nearer to 3/16 to 1/4 Inch, which would be more to the factory liking as with most production cars.

If you want to subfloor the whole trunk then resort to the above information or maybe someone else will post some other idea you may acquire......

...anyone else...?

What do you want on it?   An amp or a sub box would be easy to do.  make a box to fit either one, making sure it fits in the spare tire well.  screw it to the bottom of your MDF template.  test fit it again, then for a sub box, cut the size speaker hole you need, or for an amp, cut out the shape of the inside of the box, if you see what I'm saying.  You are more than likely going to lose your spare tire with one of these.  You can get cheap non backed carpet at partsexpress.com. 

Here are a few examples of what I'm talking about:

...that posted as one link.  They should be seperate.

http://www.k-series.com/Images/Tech/EP_SPARE_TIRE_SUB/EP_SPARE_TIRE_SUB.jpg

http://www.madpsi.net/Projects/MR2%20(SW20)/Madpsi%20-%20Pouyas%20Amp%20Rack%202.jpg