Depending on how taut the medium is at the top of the fence, you could use the bipod on the front as a modified barricade stop as well. Pope the legs down and put the robe, chain, wire, etc at the top of the bipod where it meets the stock. Press forward into the bipod taking up as much slack as possible in the barricade material and thus eliminating as much play and movement as possible. This will also puch the rifle into your shoulder and should help stabilize the rifle.
This would obviously not be feasible if there is alot of slack in the barricade material. A bit of downward pressure would help in those situations.
In the rear, I mainly just cup my hand against my chest making a 'U' shape and then just resting the base of the stock in my hand. You can grab a handful of shirt if you want a little more stable of a platform. That method is a little more field expedient.