I would use Hal Jordan for the GL,
I would make him look like this, his civilian clothes, only with a Green flannel shirt.
Hal Jordan would already have been a Green Lantern for some time now, a veteran.
To keep him usable for the show I would have him go to Earth on an
undercover mission. He has to keep a low profile,
so there would be
no costume shown. He only wears his
civilian clothes, but his shirt is green flannel and perhaps once shimmers green.
We also see his ring on his hand at all times.
His mission could be to track down an alien that escaped to Earth.
(The undercover angle would keep the cost low and keep the costume out of the picture which would keep the believability in check.)
He is about 10 years older than Clark. And he has green eyes, contacts even, and maybe they even glow green in the right light.
Hal is keeping a low profile either because Earth isn't ready to know about aliens and the GL Corps or because he has to sneak up on the alien.
But because he is he barely uses his ring and only for little things. So that means no giant over the top Green energy constructs.
Minimal Use of his ring while undercover on Earth.
At most little Green energy handcuffs, using it like a laser to cut or shoot something, maybe even a green energy hologram. It also talks to him like a computer.
Perhaps this is how he runs into Clark, his ring scans for alien life and finds a Kryptonian. There could even be a short scene where they fight, or not.
Like all Smallville guest stars, he would impart wisdom on Clark.
His wisdom could be about protecting mankind even though they don't even know about it happening, or about aliens and how mankind would react, or perhaps being a hero. Maybe he talks to him about what Hal knows about Krypton and Kryptonians. In turn Clark inspires Hal too.
There is a brief scene where Hal's ring is out of juice and he has to say the Oath, and a scene where his ring wont work against yellow.
(Maybe Hal needs to stay at the Kent farm or something for a while for some reason.)
In the end Clark helps Hal capture the alien criminal. Hal and his prisoner glow green and shoot off into space like a comet.
Thats how I would do it.
t:
