Here are a couple of update photos. The two horse limber from a 25mm Napoleonic gun will be pressed into service for the sling cart. This piece, including the horses was scratch built.
On the details of the sling cart, you can see bits of the Historex caisson wagon cut up and rearranged. The main timbers are a bit of bass wood stick and the white bits are Evergreen plastic or the Historex parts. The brown bolt head is a Grandt-line part. If you recall the two sections of the sling cart are not identical. The front unit has the reinforced wheels and has (or will have) what looks to be a bottle-jack type lift for the heavy end of the gun. The rear, muzzle-end has large wheels and has a screw type lift. The rear, right-side wheel is the scratch built one.