Up the alcohol content in Rood Beer

Howdy, I have been making root beer carbontaing in 2 litre bottles using extract, sugar and champage yeast. I know there is small amounts of alcohol in this but would like to drive it up to the level if not higher than that of a domestic year.
What would be a simple way to do this?
I have thought about using more fermentables like malt. I have a nice brew store right down the road so I could most likely find good products there for this. I have also thought about letting it ferment longer but am worried about exploding plastic bottles.

Any ideas?
Thanks in advance!

more extract or more sugar.

ferment longer.
JJ

reread your post. Put an airlck on the two liter bottles. Then use more malt or sugar but use the airlock. When fermentation is done, add a bit more sugar.. there are caluclators online to determine the amount in oz's for a given volume of beer.. and put the caps back on.

Of course you could buy some brewing equipment and do a larger batch and rack of the trub then bottle etc..