Non-frosted LEDs will make a dot whatever you use to diffuse them (unless it's very good or a bit of a distance away).
Even frosted LEDs can produce a bit of a dot but much less.
If the distance is not very great then you might want to diffuse it twice, frosted plexi or glass is normally only frosted on one side so you still can get a 'dot' on it. I have used double sided frosted plexi before which works much better, then it diffuses it, the slightly diffused light then passes through 3-5mm of acrylic before hitting the top diffused surface.
Not sure if you can buy it, I have just done it with a laser cutter before. Etch one side at a low-ish dpi and then the other side. Different powers/speeds/dpi/focus settings have different effects. Takes a bit of experimenting. This also takes ages as the laser marks ~0.1mm in each pass so if its a large area, it has a lot of passes to make!
Mowcius