I have used Windshield Washer Fluid Jugs to take up some volume before. Just make sure you strap whatever you use down...

That's probably the best idea out of all. You "kill two rabbits in one shot" right there. :)

As long as you're not operating outside of the capable parameters of the driver. "Recommended" specs are a good starting point but still leave lots of room for error.

It's not really a question of whether it's a 10", 12", etc, it's more determinate on the specs of the driver.

If you want maximum performance out of your driver you need to take your T/S parameters to formula. If I am looking for peak performance, as a rule I always spec my box to the driver and never try to make an enlcosure that was designed for a completely different driver work.

Ported boxes are not as forgiving as sealed enclosures either. You run more of a risk of running into flabby bass if you stick it in an enclosure with more volume than the driver is capable of displacing for Fs.