The trunk kit from Factory Five was designed for the 818S, but I wanted to have some storage space in my 818C, so here's what I made.
trunk composite.jpg
It is close to the downpipe and muffler, so I made a heat shield. The trunk is supported by a steel frame of square 1/2" tubing anchored to the transmission frame. I lined the inner walls with 1/2" closed cell foam for heat and sound control, plus D-rings and bungie cords.
trunk composite 2.jpg
A fun creative aspect of FFR builds is that we each can design and build what we want. My trunk is aluminum in 3 parts so I can install and remove it with the hard top in place. Insurance Guy (Alan) made a one-piece fiberglass trunk for his 818C that he installed before mounting the hard top. Alan has the FFR trunk to use when he converts back to an 818S roadster. See his well-made fiberglass trunk in the build thread for Insurance Guy.