I realised too late, you can make one yourself with a softdrink bottle, either cut the funnel top off and invert it, or leave it as it is. The amish I believe just use bottles or jars with a few holes poked in the lid. Apparently the lower oxygen level inside the bottle works quite well to ensure no escapees.

BTW - I"m not a mass-insect murderer, usually try to leave them to their own devices, but we had a sewage incident in our yard earlier this year and the flies all flock to that area even though the soil has been dug out and replaced...I am treating the area with enzymes but they will take time to work and I just can't let go of the idea the flies could be tracking poo fragments everywhere in our outdoor eating area, etc.