Yea I've had the same light spill problem. Not exactly sure who to look for if you need someone do do it for you. But if you DIY, it might be a lot cheaper. eg. one big board of 2440 x 1220 is about $35 if I recall correctly, and the shop can cut it for you $1 per cut. http://dama.com.sg/product/pvc/
I DIY-ed mine and happy with results. You can use acrylic or PVC foam. I used PVC foam 5mm version as PVC foam is much lighter and you can use PVC glue to bond so it's pretty secure. I ended up only using less than half the 2440 x 1220 board.
For mine as its a long fixture I added hinges (strong double sided tape to attach) for easy access. See pic.