Not sure who could build you one, but, I have had good luck cutting plexi right on my table saw. I just make sure to lower my blade down so it just barely cuts though it and then I round my corners with a belt sander.
If you want to be sure to get nice exact corners on both sides, cut a curved template and clamp it to each corner and sand up to it, or, if you have a router and no sander, cut your template and route plexi with a flush trim bit.
As for painting it, if you are only doing one colour, it is impossible to screw up. Just remove protective film from the back, spray a few light coats until fully covered, wait till dry, install and remove front film.
Good luck
