Vignetting with nebula filter

When I use my Orion Ultrablock with any "max field of view" eyepiece (i.e., 27mm field stop in 1 1/4" format), I get a dark, fuzzy edge of field. Is this going to be inherent with any screw-on filter and a max-fov eyepiece, or are other filters designed to give you that last teensy bit of clear field?

Since the inside thread diameter is about 28.5mm, anything you screw into it will of necessity obscure the edge of that 27mm field stop. But why does the filter have to be screwed in? It will also work held in front of the eyepiece between it and your eye, and now you have no vigneting.