with the multi-chip fixture, all you need to do is find a 2'x4' sheet of diffuser from lowe's, look for plaskolite brand (it has a green/white sticker) in the area where they sell ceiling tile for 2x4 grid ceilings. They are about $5/sheet but if you find one that is cracked (there's always one) they'll usually give you $1 off.
You can cut it with a table saw, but it'll throw chips everywhere unless you use a high-tooth blade. The easier way is to lay it on a table and put a straightedge on it, and score it with a box cutter, use a sharp blade (new one) and make 3 or 4 passes with medium pressure, eventually it will cut through. Score & snap will throw shards everywhere so just score, score, score.
You can cut it to fit the entire front of the fixture and then just tape the edges with duct tape or electrical tape, or you can cut a small piece to just go over the middle, maybe just 1 or 2" square.
The other thing you can try is to take blue painter's tape and lust place a small square on the splashguard directly in front of the chip. This will significantly cut down on the light intensity right in the middle, but the screen will still get adequate light everywhere. That's probably the easier way to try at first.
