How do I keep epoxy from leaking out of cracks that go through the wood?

Block the underside with a rigid, removable backer before you pour. Use a sturdy, flat, level board such as thick plywood, MDF or a 2x4, and cover the entire face that touches the epoxy with epoxy edge tape. Any bare wood the epoxy reaches will bond to it and the backer won't come off cleanly.

Seal the seam between the backer and your piece with silicone caulk so the epoxy can't seep through the joint. Keep the silicone off the work surface itself, because epoxy has a hard time adhering where silicone has been.

Let the epoxy cure for 3 days, then remove the backer and tape.

Pick the fill product by depth: cracks 1/4" deep or more are filled with UltraClear Deep Pour Epoxy; shallower cracks with UltraClear Bar & Table Top Epoxy, never more than 1/8" per layer. On a table or bar top, fill the through-voids first, let them cure, scuff-sand and clean, then seal coat and flood coat the whole surface with Bar & Table Top Epoxy.

Temporary dam guide: https://ultraclear.helpscoutdocs.com/article/223-temporary-dam-epoxy Epoxy edge tape: https://www.bestbartopepoxy.com/products/epoxy-edge-tape Silicone caulk: https://www.bestbartopepoxy.com/collections/tools-supplies/products/silicone-caulk