camo nets over tanks or howitzers etc.

For poles, depending on the scale, try either stretched sprue, sprue, or buy some plastic styrene rod with the desired diameter.  Many times, netting is placed utilizing field expedient poles (trees, logs, branches, etc.) in addition to the poles.

For the netting, I agree with using gauze (one 4" x 4" gauze pad makes a large 1/35 scale camo net if unfolded) or cheesecloth. 

As for positioning it correctly, try using a block of styrofoam similar in size to your dio base (or better yet, use styrofoam as your dio base, then cover it with your plaster or ground-making material).  Set your poles with spreaders at the desired angles. 

Method 1) Dampen your net material (gauze or cheesecloth) with a thinned (thick soup consistency) white glue & water solution and drape the net over the poles, temporarily tacking down the edges at odd angles.  Once it dries, the glue will generally keep it the way you want it.  You may have to strengthen it with another coating of not-so-thinned white glue before painting or handling.  You can even add the paint color to the white glue solution initially and/or follow-on coats. 

Method 2) Drape your dry net material over the poles, then spray with fixative (hairspray, paint, spray glue, etc).  Let dry, the paint. 

If you're looking for sources on camo netting and what it looks like.  Google Image Search "camo net", "camo net position" and "camo net artillery" and you will get some pics of artillery and armor positions camoflaged with netting. 

Hope this helps! Keep us posted!

What scale are you working in? For WWII era nets in 1/35 a good way to replicate is using gauze or cheesecloth. The nets themselves were usually a large mesh with lengths of burlap woven through in disruptive patterns. To simulate this, paint the net a lighter color, green or tan, and paint a darker disruptive line pattern. At the top if the pole is gonna be a "spreader" x or y shaped  cross member to support the net so that the pole does not go through the gaps in the net. Poles will be placed at angles and edges of nets staked/tied down to give an irregular shape.