Mike;
The best piece of advice I can give you on making these is good reference photographs of the type of cover you're replicating. Once you have these in hand and have studied the basic shape of the cover you then have to choose the method that suits you.
1. Tissue and white glue works quite well and is the most readily available and cheapest method to use. Begin by drawing the basic shape on a piece of paper or thin cardstock. Add a slight bit of extra to all edges for any covers that overlap the outer edge of the intake/exhaust. Take a single tissue (I find the "industrial" type that are like sandpaper to blow your nose with are the best for this) and find a box (old kit box or similar) that is slighter smaller than the tissue. Drape the tissue over the box and tape the tissue down on all sides securely (it's like making a rectangular drum) once done mix white glue and water together in a container and brush over the surface of the tissue. Allow this to dry fully or use a hair dryer to speed up the job. Continue applying the white glue mixture until the stiffness of the tissue is what you require. Once this is fully dry remove it from the box and using your template of paper or cardstock cut out the cover from the stiffened tissue. Place it in the area required and secure with more of the white glue/water mix. One final step to "disguise" the tissue texture is to use a few good brush coats of "sprue goop" if you've never made it... try it. Take all the leftover sprue from any kit and chop into tiny bits. Drop all the bits into a bottle of Testors or Tamiya Liquid Cement. This takes a few days to dissolve the sprue bits. What you will be left with is "sprue goop" paint it on with an older flat brush even strokes in the same direction and this will "fill" in the texture that makes tissue show like tissue and give you a closer to scale canvas or tarp look.
Then you paint as required.
With all of the above said.
2. Lead Foil is the materiel you're looking for. It can be found at the local hobby shop in good craft shops etc. I prefer to "recycle" in my modeling and my source is wine bottle foil. I have been collecting it from various people for a few years and it works great and costs nothing. Once you have your foil from a source. You can use the same method above as with tissue paper by making a template first and cut out from the foil with a good sharp pair of scissors. Secure with epoxy or super glue. Prime and Paint.
The other way is a lot more involved but you could make a copy of the nacelle or exhaust and vacuform a cover over top and get an almost precision fit.
That's all I have to say on that. I hope it helps you.
Cheers;
Gregory