Here is the stand that was used to hold the cannon.  It uses a set of springs to help withstand the huge recoil of the cannon firing.  The springs are attached to two cables, which are attached around the 4"x2" bushing.  To prevent the cannon from springing out of the stand after being fired (reverse effect), I made a homemade hose clamp out of some plumbers tape which holds it in place quite nicely.

You can see the springs and the cable.  The cannon sits inside the cradle shown below: