Do you mean the Skid Plate doesn't fit perfectly? That's pretty normal. If the case of the coil itself is open or has a gap, this is abnormal. Don't use it if there's an opening directly into the coil housing. Return it. I think that what you have described is the coil cover (scuff cover) being ill-fitting.
They all seem to fit a little poorly on this coil - or at least they look looser than they are.
This is the case on every 5x10" elliptical I have seen.
I wouldn't seal it with silicone since it's designed to be removed when damaged. Replacement is only around $10-$12
But you don't want moisture and magnetic or iron particles getting in there ...and they will. The iron particles become magnetized in a short time
from the coil field and then give constant false signals to the coil.
Use Black Duct Tape to seal the coil in place. It looks very neat and will prevent the coil cover (which will eventually crack if you hit
a rock hard enough) from falling off. Nothing will annoy you more than a split coil cover constantly falling off in the field.
Here's what I did with my own to keep it in place - Note the tape is wound twice around the rim:
