That could work as a way to affix it and possibly the other diffuser I mentioned as well.

The second idea - bulge down - is backwards and likely not to be too satisfactory. I have an elliptical optic to make the magicshine's throw into a wider beam (see this thread). The optic is similar to that lens but with finer ridges or striations.

Using the 501B, I rested it face up then face down on the lens and did a ceiling test. Face up, it kind of works, but it is so far from the LED it doesnt create the proper pattern - like a focus problem. Face down it is worse - makes a fine horizontal line (like trying to use a telescope for a microscope). With all the light going thru all the optic, it is noticeably dimmer and has artifacts. The soft optic had similar results.

The wider striations in DX1918 may make a big difference even backwards though, who knows...

The stock 501b is definitely too spotty/beamy and needs something; and if it throws some of the light to the sides all the better.