There is an inelegant and non-tech way of doing this, but it is very easy.
Do you have a standard Dex receiver?
Set your low alert level to the highest possible setting on the receiver, probably would be 100 on the receiver.
Set your high alert level to the lowest possible setting on the receiver, probably would be 120 on the receiver.
Turn all the repeats to 0 minutes.
Turn the “urgent low soon” warning on.
Turn the rise/fall rates to be the lowest values, 2 mg/dL/min.
Put the Dexcom receiver were you can see it, like on the dashboard or on your visor or something.
As long as you do not press any buttons, your BG will appear every 5 minutes (as long as your BG is not between 100-120).
If your BG is between 100 and 120, it will not appear, but so what? Your value is fine.
But as soon as you get to 99 or 121, it would start popping up and displaying every 5 minutes as long as you do not acknowledge the warning alarm by pressing a button.
This is pretty much a hands-free system. Very low tech but completely functional and very easy to implement. And since most people get a receiver with their initial Dex script, you can probably use the receiver you already have handy.
I know this seems low tech, but if you have a receiver, this setup costs nothing and takes about 2 minutes to setup. Try it sometime.
The key point to repeat - as long as you never acknowledge the alarm, it will keep showing up every 5 minutes for any value other than 100-120, or for any fast rises or falls.