Sturdy Gunpla Runner Stand

A remixed version of "Sprue Holder for Gunpla & Scale Models".

First off, the original design is excellent! I really appreciate how these stands let you move runners anywhere you like, do not take up table space once your build is done, and require very little storage. The only issues I ran into were:

- The stands were a bit light, so taller runners would sometimes topple over.
- The vertical edge of some runners did not always align well with the 3 point grooves, which could cause leaning (leading to the toppling problem).

To address this, I remixed the design for the "Sturdy-Runner-Stand" with a few key improvements:

- Added a central post for stability and increased the height slightly to address the leaning problem.
- Included six slots with varied widths (2.25mm, 2.5mm, 2.75mm, 3.0mm, 3.25mm, and 3.5mm) to fit nearly any runner thickness (some peg trimming may still be required for to achieve a snug fit).
- Designed the slots for a snug fit, so the base grips the runner. This