First of all, nice organic flow of the chassis. I really dig this idea For the plexi to cover up the holes, well... I'd go with soaked fibreglass mats and a baloon like in your picture and have it find it's own shape there. Afterawards simply sand down and paint the fibreglass glossy black. Another good material to work with and achieve good results without alot of trouble would be thin sheets of plastics, usually used to vacuum-form moulds with, and aswell simply paint it afterwards. Heating these sheets up is no problem with a heatgun on low temperatures. Plexiglas simply isn't very formable other then to bent it on a straight line.