Drag to orbit. The holes are open at the top and blind at the bottom — each one stops short of the underside, so the rack has a solid floor.
How the holes are sized. Each hole is a plain round blind pocket, cut straight down into the base. Its diameter is the tube class's own outside diameter plus a deliberately loose clearance — a test tube gets lifted out and dropped back in by students all day, so this is an easy slip fit, not the snug press fit we use for parts that are assembled once and left alone. The rack's footprint is the same for every tube size; the base works out for itself how many holes of that size fit in a grid, with a solid wall left around the outside.
How deep the holes go. The holes are not the full length of a tube. They go down far enough to hold a tube upright against tipping while most of its length stays exposed above the rack for grip and reading — the same way a real bench rack holds tubes. Beneath every hole a solid floor remains, so nothing can drop through and the rack stands flat.
What this is. One printed piece, no lid and no moving parts — a plain storage rack for a classroom, teaching lab, or hobby bench, and nothing more.