What I usually do for panels that are a close fit, is to
1) draw the shape of the panel.
2) Then use Trim to cut the surface.
3) Extrude the new surface that you have trimmed, back into the body.
4) Then go back to the body and extrude the edges of the new hole back.
5) Join the new faces you have extruded to the body.
6) You can the place a small fillet round the edges of the new solid you created in the first extrude and the same radius fillet on the edges of the opening in the body.

This technique works on curved surfaces as well.

If you want the original body to remain a solid, it is a little more involved. Go up to step 3. Then select the trimmed edges of the original body and delete them. The hole should disappear. Then boolean difference your new solid from the original body, making sure you tick the "Keep Objects" box. You will then have a recess cut out of the body that your panel will fit exactly. Continue on with the fillets.