I made a flie trap, I put some meat inside and after two or three day there were flies inside the trap, about 6 or 8 flies eache day, but I could recollect them for 2 or 3 days, then the flies didn´t get inside the trap, but I saw maggots inside the trap and thanks to PhilinYuma I could get some pupaes and then Flies.
The trap I made was like the next figure
some weeks after I used the trap again, but in that time besides flies a lizard felt inside the trap and it couldn´t get out.
So I can suggest to hang up the trap some inches above the ground if you want to make one like this one.
saludos