Take a deck to home depot, and If you don't have a spare deck just take your whole scooter. Go find some metal that will fit well (width wise). Feel the elasticity of it. If it's pretty wobbly, but doesn't bend all that much, then it's perfect. Buy it, then once your home, just estimate on the length you'll need (cause you can always make it shorter later. Then put it in a vice, and bend it to the shape of your wheel.
If you don't have a vice, then drill the hole in your deck and the flexy, bolt it on, bend it almost straight up (so you can fit your wheel in), put in your wheel, and shape the metal to the shape of your wheel.