this is for creating sand cores for the foundry, core sizes are 4 inches long, and vary from 1/4 inch wide up to 3/4th inch. I designed this to cast out of aluminum to create a half of a mold, but can be used as is, but need to print two, or cast it twice.

Sizes are 1/4, 5/16, 3/8, 1/2, 5/8, and 3/4 inch.

Just print two out, or print one and cast it out twice, clamp together, and ram your core mix in with a dowel rod.
There is a bit of room on each side of the print to drill a hole through both to insert a locating pin.