I've concluded I may as well put a speaker back in it (I had considered gauges or something), so I wondered what you guys have done.
Originally is was a 4"x10" mono speaker, have any of you put a modern speaker in here?
The most popular options I'm aware of (and it's taken a lot of looking!) are either putting a couple of small (3.5" or 4") round speakers in there, (although it's pretty tight in a Challenger dash, so I'm not convinced this would work easily) or using a Dual Voice Coil speaker. You can get these from Custom Autosound in the US, they have stereo tweeters (i.e. 2 of them!) in a single 4x10 speaker.
Seems to make quite good sense and they claim that the bass is much better than two separate speakers.

I haven't figured out the rest of the system yet (not exactly top of the list!) but all it has to do is be loud enough to hear with the top down, decent quality but nothing fancy. The same company do a couple of under-the-seat subwoofers which seem like a great idea, mainly because they use redundant space, and there aren't many options on a convertible.
So before I order up one of these DVC speakers from the US, any other experiences, comments or suggestions, I'd appreciate it!