Those CLD products (constrained layer damping) are mostly intended to be applied with partial coverage to stop panel resonance - by adding constrained mass to the panel which stops it from resonating at the nuisance frequency.
To stop transmission of outside noises through a panel (such as exhaust or road noise passing through a panel into the cabin), you would need several layers of CLD product applied as seamlessly as possible, like Jeff mentioned. CLD is probably still the best product for the job, due to its ability to be rolled into the contours of the panel and then have the factory finish (plastic panels, padding, carpet) placed back over it for a factory appearance.
On a sports car there is typically less or no factory sound deadening compound applied, to save weight. Sound deadening compound on a floor pan for example would be a thick layer of tar-like substance applied to the whole panel, which then is concealed (and aided) by padding and carpet.
It sounds like you would need at least a few layers of CLD product applied in full coverage to your trunk floor to net the same effect. Use a roller to get it into the contours of the panel, try to butt your seams as neatly as possible, and foil-tape the seams on the last layer to keep the goo constrained.
I bought some "Noico" brand (search it on Amazon) 50 mil butyl rubber CLD product for one of my cars but have yet to install... has great reviews though. Looking at Amazon now, it seems mostly 80 mil thickness is what they sell now. The Noico stuff has a pillowed appearance to the foil, so you know when it is properly rolled into the contours of the panel (the pillowed foil flattens out).
One of these days I'll be done (or done enough) with the theater to get back to my car projects....