I have a Polder thermometer that's similar to this:
http://www.amazon.com/Polder-Cookin...k/dp/B00004S4U0

That'd be ideal for DIY sous vide!

Cooking longer is ok. I just would be worried you can't get the temperature low enough. Even on my lowest setting on the stove top it seems like the water is just at a simmer. I don't know. Maybe it wouldn't get that hot if I didn't turn it up to start with. Hmmmm.... If you're cooking beef and the temperature is too high you might not want to cook longer.

I wonder if maybe the lowest setting on your oven might work?

Also, you can take the internal temperature. And because it's a roast, any bacteria is likely to be on the outside so just make sure the internal temperature gets to what you want, assuming you use ziploc bags, that shouldn't be too difficult. Although you generally want a longer cooking time for a chuck roast to break down the fibers.

Well, I'll be real interested to see how it turns out! I haven't done a roast yet!