Edog, I'll try that technique! It sounds good! My way, so far, anyway, has been to make the tarps and other bags out of Milliput epoxy putty. It works very well too.
For flat/folded sheets (and flags), there's a way to work out very thin sheets of the stuff, by rolling it with a rounded object and using lots of talcum powder (or baby powder).
For bulkier items, create the basic shape with your fingers (and a bit of baby powder) then use a sharp knife, or a toothpick, or else, to 'imprint' the straps (by pressing the edge of the knife, for instance, along where the edges of the straps would be).
Before the stuff dries (Milliput 'normal' takes a few hours to do so) add the tarps/bags onto the model and gently press into place (I use a piece of cloth to do so, in order to avoid any fingerprint, and this also give a bit of 'fabric texture' too). Wrinkles can then be added with other tools. Let dry. Then either let dry and paint/weather or pop the bag of the model (would not work with thin items, cause they'll break!) and paint/weather before re-positioning with wood glue.