The problem with small loudspeakers like these (this one is 50 mm - 2 inches - across) is that there is no mounting holes for fixing it to a baffle. I've devised a simple way of overcoming the problem.
Step 1: What You Need
1. Two eyelet screws (the kind you use to attach hanging wire to the back of picture frames).
2. A strip of stainless steel, salvaged from an old car windscreen wiper arm.
3. And of course your loudspeaker...
4. ...and the baffle you want to mount it on. I assume you've already made the right size of hole in it for your particular speaker.
Step 2: Use You Drill
Drill two holes in the baffle as shown in the picture, one size smaller than the thickness of the eyelet screws. I've found that 10 mm (3/8 inch) from the loudspeaker opening works for 50 mm (2 inch) speakers, and 15 mm (5/8 inch) works for 40 mm (1 and 5/8 inch) speakers.
Step 3: Screwing Time
Screw the two eyelet screws into the holes you've just made, so that their openings face each other.
Step 4: Now Do the Strip
Cut off a piece of the strip of stainless steel, a little bit longer (say 2 cm - 13/16 inch) than the distance between the eyelet screws. A side cutter works quite well.
Step 5: Fit It All Together
Push the strip of stainless steel through one eyelet, bend it over the loudspeaker, and pass the other end through the remaining eyelet. I've found that the loudspeaker is held quite securely. The speaker doesn't get damaged at all, and it is very easy to remove it again. If you're a bit paranoid, you can of course dab on a blob or two of silicone sealant to keep everything in place.