For laser cutting would like to stick to Mild steel. Laser cutting aluminum is different story all together. As swapnil said 1.5 thickness is a good choice.
You can design it using Fusion 360 and export it as DXF file. Its easy to learn - you will find plenty tutorials on youtube. If not ask them to make a design for you.
Many laser cutting shops use CDR (coreldraw) files to generate gcode. But since coreldraw supports DXF (IIRC) and they can take care of it.
Autodesk Fusion 360 is free for Hobby use. You just need to renew you free license every year. I think Fusion 360 is one of the biggest contribution to the DIY community by Autodesk. DassultSystemes must learn from Autodesk