Wow, step away for a few weeks, and I see a whole community is forming! Haha!
There are certainly lots of ways to skin this cat. Maintaining temps isn't necessarily rocket science, but it's a lot of fun when you decide to throw a rocket at it! Wayne, I'll be curious to know if the aquarium heaters do it for you. I feel they are sort of weak, and are probably better at maintaining than at initial ramp of the temps. Also, if your container isn't insulated, they might have a lot of work just dealing with the dissipating heat as the surrounding air tries to cool the container.
If you have a big tank, I think your original big heater would be just the ticket. But just make sure you have a strong circulation pump to keep the hot water moving off the heater. If the SSR can drive the heater (big question, make sure it can handle the load), it should cycle rapidly enough that the heater will never get very hot except during the initial heating. Mine cycles at a blindingly fast rate.
My PID controller has the ability to set temperature alarms. You might check that, if you fear the thing getting stuck on. Or, get a separate electronic probe with an alarm if you really want assurance. So far I'm not feeling the need, but I can imagine if you're D&D with big 8x10 sheets you might want some extreme assurance!
I love the simple roaster idea, but for the $ spent, I'm happy with my way-too-accurate bath...