Odd observation.. A XR-E, a MT-G2 and a XM-L in series and 2 are dark

You can also use water bottle caps as diffusers, I often use them set overtop of my bench test emitters so I don’t blind myself looking right into them.

I've tested all three ideas now.

Just tape directly. Little effect.

Several layers directly. Little effect x No. of layers.

A few cm away on glass. Little effect.

Bottle caps. Effective but absorbs a lot of light. The only ones I had around are not really that transparent though.

Solution. Several layers of tape :-)