An alternative to milliput (a better one) is kneadatite, also known as green stuff.
Table top wargamers use it to make cloaks, banners, and other similar items, even whole models. The more traditional side of modelling, i.e. what we discuss here, seems to be rather behind in this way.
It can be rolled/pressed very thin so you could make use it to make a 1/72 scale tarp.
And I found this which may help - http://www.network54.com/Forum/47210/message/1338861202/Green+Stuff+for+tarpaulins