Found this trunk liner in JC Whitney catalogue a year ago. Since then it's disappeared
from their catalogue. Finally found my paperwork though and called them about it.
They can still get it and price is still $79.95. Made by a company called Neocon in
Canada. JCW is at 312-431-6188 in LaSalle, Illinois. Stock # 94-7948-P.

This gets the idea across. Keeping water (especially the salty type) out of the trunk.
This is a near perfect fit in my 97 2.8L Z3. Just enough room for roll of paper towels
on left side and a whatever you put in the plastic well on the right side. CD changer is
fine except need to leave off the cover's front flap. Also spare CD cartridge can't be
removed from it's holder without lifting up a bit on the liner.

This and the next shot show the result of the easy modification.
A hack-saw blade applied judiciously to the lip of the inner edge
so as to remove exactly 1.25 inch. Be sure to sculpt a slight curve
around the corners, as a sharp corner is vulnerable to tearing. I'd
already boought the standard BMW trunk liner which is better than
nothing. This one fits so well that it's a little tight when placed on top,
so leave the former one out. Also, the slight dip in the front of the
trunk means a little padding will provide support for heavy items.

The liner measures 8 inches high x 21.5 inches deep (after mod)
x 37.5 inches wide in back and 34.5 inches wide in front (the
modified end). Even though it cuts pretty easily, the plastic is real
durable and about 3/32 inch thick. Also, it comes with partitions
that fit into those slots.
update 10/14/07 with current space in my trunk with trunk liner