How much of the seat post is sticking out of the frame?
If there is enough you can find a piece of tubing that has the same ID as the seat post's OD and cut a length equal to the height of the seat post.
Now cut the short piece of tubing in half lengthwise. Securely weld the half tube to your wire cage and use hose clamps to hold the half tube to the seat post tube.
You can also get creative and use small pipe clamps (minature muffler clamps)
welded up to the half tube and have an even more secure clamping system.