Just posting this thinking it might help some one. First let me say this is a project of pure curiosity. I will probably try this on making bunkers when I get the chance.
Okay, first I sized a picture I found on the internet, glued the cut out to 1mm sheet plastic, and trimmed it to form a "scrapper" of sorts.
Then I went to the local hobby shop, that also frames pictures, and bought their scrap matt board. I wound up with a large grocery bag for about $5.00. Great stuff, cuts easily with a hobby knife (I used trauma shears for the larger circles). Used regular white glue and a small 10 pound weight to keep it from warping.
Next I used DAP Spackling Paste and shaped it. It took about 3 coats due to the shrinking. The matt board did swell slightly, but it returned to shape as the plaster dried.
Sweet thing about the paste is that it can be "sanded" using your finger or a regular nail file. I used enamel spray paint for 2 coats to seal the model. It ended up being pretty tough, but I haven't dropped it on a hard surface yet
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Here's the first two (now working on Haunebu 3. It's about 25cm in diameter). Not satisfied with the camouflage. Might just repaint to gray. Need to get a decal sheet on my next GHQ order.
Off topic. I remember a thread discussing storage of our armies and this is what I've done after visiting the local hardware outlet. Used heavy duty shelf glides and plywood covered with black cloth (hides the dust
) I need to add about 4 more shelves one of these days. Sorry about the focus, but its good enough for a general idea.
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. Just have to wait until some one makes them (checking into ZZ scale / 1:300 trains for vehicles or animals. I'll pass on any info I find). 
