The more time I spent trying to mill what I needed (a square hole with a 30 degree angle at the bottom), the more I realized that I needed to find something off the shelf. And sure enough: I found some square and rectangular aluminum tubing the dimensions required for this project. I can plug the bottom of the tubing with a piece of acetal tapped to accept casters, and use machine screws to lock the plug into the tubing.
A few rules for machining stuff:
1. Machine the last .050" inches; use coarser tools to get closer. A chop saw gets you to 1/16" of an inch. I have a planer that lets me get down to .1", easily.
2. Never try to machine something from scratch if there is a part available off the shelf that is close to what you need. The time that you spend trying to machine a block into any shape except a square, rectangle, or a circle, is going to be significant. If you can find that shape off the shelf, whatever you spend on the part will be worth it in time savings and frustration reduction.
3. Always look for a way to machine the part while it is still simple enough to easily clamp to the table (usually because it still has a flat surface). This often means doing something out of the logical sequence.