White Glue Intake/Exhaust Covers

The easiest way would be to carve solid masters of the exact shape of the intake exhaust covers and then form the soaked tissue paper over them, basically the same way a vacuum formed plastic kit is molded over the masters. Then they can be removed, trimmed and painted.
I've had fairly good luck with white glue soaked tissue for bedrolls, tarps and fabric seats and roofs, but I've found the texture a little rough for jet covers - the covers I've seen in real life were themselves thick vacuum formed plastic of some kind, being very smooth.
But then again, I've seen some pretty good renditions of the covers on some of the pictures in FSM -
Good luck
Lee Tree

If I can add to what Lee say's, I've used the method on a 1/35 army truck in the past. I just used ordinary cheap toilet tissue & diluted white PVA glue, Elmers would be fine, diluted about 50/50 with water. Try covering the intake/exhaust in plastic electricians tape then apply the tissue & glue, it should'nt stick to the tape, but I imagine you can peel it off when it's tacky as it will still bend a bit but won't lose the basic shape. Paint as required when dry, and it dries fast!
Might be an idea to try it on an old kit first though.
PVA glue makes a great paint on gap filler on kits, & can be used to stick down canopies, by the way.