a few other points to consider:
1)If the sub woofer is a ported model, than the port must be on the downward face to allow the wave from the port to vent properly. Venting the port into the trunk will only cause more rattles.
2)If the sub woofer is a powered model, you must have signal and power at a floor box located below the trunk, but not directly below the sub woofer (a simple thing, but how many times does that simple thing that gets missed lead to grief?)
3)If the sub woofer is a powered model, is there a way for the built in amplifer to vent the heat generated in the amplification process to avoid thermal shutdown; or does the trunk have enough internal volume to allow for this factor.
4)Does the client understand that his antique trunk will be modified?
5)Which type of client is it? sonic perfection or cosmetic integration? Either way, there are trade-offs that certain people are willing to make while others wont.
Just a few things to consider before proceeding.