What bothers me freind, is the 'should be' part. Who says? Yes its a common practice, but there are no feeding cups in the wild.
I beleive its important for lizards (not just chams) to able to exhibit natural behaviours, including hunting.
As far as the cham is concerned, Ive read from just as many 'experts' that its favorable to allow them to hunt naturally (regular use of the tongue mechanism) maintains the muscles apparently, and from the same folks perspective, more tongue damage occurs with feeding cups than without.
Personally I find a shallow cup hung below the basking spot is great for some feeders like worms, and my cham atleast still uses his tongue without issue (but gets much closer top do so). For crickets I alweays free range, Homer remains active and gets full use of that magnificent tongue, and seems to enjoy hunting as he would in nature.
The only concern with free ranging, is that uneaten insects can begin to chew on sleeping lizards. Care should be taken to carefully regulate how many are provided at any given time, so that as few as possible remain overnight. Personally I provide a cup of fresh greens on the floor and find any lucky survivors of the day, remain there eating/hiding, well away from my sleeping cham.
Cheers
P.S regards concerns of chams trying to shoot insects through the sides of cups (or below), I really dont think its a great issue with a little forthought. I dont think it matters what color it is (though neon bright colors might upset), or even clear.
A wider cup might be better than narrower (must be more effort required for a cham to focus down into a narrow dark cup no?)
With repetition, most will learn quickly (especially since food is involved). A cup hung or attatched so its lip is below or level with the basking spot or favored perch, means the lizard is looking down into it already. Placing it while the animal is there (if poss) avoids some initial confusion, and repetition (same place each time) soon does the job.
Not too many insects at a time either, means easier choosing a target.