coreboxes and bushings

After having much difficulty using the other corebox that I previously made, I decided that it would be easier to just have 2 or 3 coreboxes for the sizes I really needed.

There is 3 sizes, 1/2 inch, 5/8 inch, and 3/4 inch. The holder_corebox, print two and you'll probably have to file on them very slightly till the coreboxes can be slid in and out while being still snug. Just slip those over each end and it'll hold the coreboxes together while ramming the sand in firmly.

There is also tw