Fresnels are for eliminating hot spots from bright objects and opening up the shadows as viewed on the ground glass which may or may not improve focusing and with a loupe makes it more difficult.
A fresnel placed between the lens and ground glass will cause the focus to shift so that the sandwich has to be repositioned to match the film plane focus.
A fresnel placed over the viewing side ground glass will make the shadow detail easier to see while eliminating the hot spots to a degree. It may make the overall brightness higher.
I have successfully used a office supply store fresnel on 4x5 cameras. A fine tooth saw or plexiglass scoring tool can be used to cut the fresnel. When scoring score from the smooth side and score half way through before trying to snap it.
A pair of 2 to 4 power reading glasses may do more to make focusing easier than a fresnel.
I agree. I put a cheap fresnel ofer the ground glass of my 2¼ x 3¼ RB Graflex, and that diminished the hot spot. What really made it easier to focus was the fresnel used in combination with a 2x pair of cheap reading glasses.