The way I use the MG filters in my Opemus enlarger is I have a set of above the lens filters which I had to cut to the size of the filter draw, Master the art of the test strip, and remember that with the MG filters grades 00 to 3 1/2 use the same exposure and 4 to 5 increase the exposure by a stop, as far as the makes of MG filter, the widest available are made by Ilford, and you can get sets for above the lens in 2 sizes, get the smallest size and cut to the size of the filter draw, or get a set of below the lens MG filters, which are much more expensive, and fit in a holder mounted on the red filter spigot, I find the above the lens works fine for me.