Ok!!
I had similar problems with an earlier build and got around it by using one of those 1/4" threaded inserts ( the type you hammer into wood to give a thread) filed of the tangs and then expoxied it into a 8mm hole in the sheet metal... Gives about 8mm of thread and support to the screw... maybe a bit late advice for you... but may help others.
You'll probably find, depending on the grating, that you have to use it everytime! To go from zero image to the spectrum! Maybe a disk of milk bottle plastic would help??