I would try and design the enclosure to where it is pushing the sound into the interior and not keeping it mostly in the trunk. I've seen (and heard) WAY too many trunk lids rattling from having subs in a trunk and from the speaker(s) moving in and out, also making the trunk lid move up and down causing unwanted sounds. The trunk lid is only attached in three area's (two hinges and a trunk latch) and even with a trunk lid closed you can still push it down some more and let it spring back against the latch due to the weatherstrip material. I did a google search for Chevelle trunk lid rattles and there can be problems if not done properly.
When I can I start with my speaker baffle board that will have the surface built up (or have things recessed into) to where I can easily get to both sides of it. I normally use speaker cloth to stretch over the forms and use staples to secure the edges or at particular detail points. I then mix up some resin and catalyst and using a cheap paint brush, put the mixture onto the cloth and after a while it will soak in and by the time I get back to the spot I started the mixture is starting to thicken and I can apply more resin to the cloth giving it a more glossier finish and less of the cloth texture being seen. I then let this dry and then come back with a second coat on the outside seen portion of the baffle. When this fully dries, normally the surface is pretty smooth and to limit my bodywork/filler work later I then flip the baffle over and then apply matting with resin to the backside of the baffle. Doing it this way I keep the smoother outside surface intact to where I have less work later and it's closer to what I want in the end.
Sometimes I cut a bunch of patches of matting to go onto the inside and sometimes I cut strips. I've even chopped up some matting to make a slurry and be able to use that in spots. You just have to watch out you don't apply too much at one time as I have had things warp. Sometimes you get into a learning curve which can be good the next time you do something.
Touching back to subs in the trunk is I've seen trunk lids open with the subs and amp racks looking real nice but the trunks are finished on all sides and while it may look good and sound good with the trunk lid open, the trunk when they close it starts to pulsate up and down and causes undesirable sounds. The trunk lid now becomes the 6th side of a box and being somewhat flimsy in it's materials and attachements, can creat undesirable noises. The inner trunk has a bunch of countours in the metal to strengthen it but the outside skin is not near as strong and is easier to flex. GM years ago put globs of adhesive between the inner trunk lid structure and the outside skin but after time sometimes this bond fails. I've also seen some trunk lids without any bonding material. A person could with a long enough nozzle tip be able to apply a glue to go between the underside of the outside skin of the trunk lid and then the topside of the inner structure but the surfaces would have to be clean and not have 30+ years of dust or whatever on the surfaces.
Here's an Impala with bad trunk lid flex:
All that energy moving the trunk lid up and down should be sent into the interior where you are listening to the music.
Here's one that probably doesn't put pressure into the trunk area but forces it into the interior:
I guess what I'm really trying to say is design it right because if you don't, or don't care, it would be like having no header gaskets on the cylinder heads and thinking that sounds good. To me that doesn't sound good at all. I would rather hear the good sounds coming out of the tail pipes as it should be.
Jim1968 Chevy II Nova Garage Find 2012
1973 Nova Custom
1974 Spirit of America Nova
1973 Nova Pro-Street