What scare are you building the trapezoid? Are you really intending on it being able to support approximately 100 pounds? Are you wanting it to be a pyramidal trapezoid or more like a ramp?
Either way, I would begin with a 2x4 square structure then build the triangles for the sides off of the structure to guarantee as much uniformity as possible.
If you want it to support body weight I would be sure to use cross-members on the inside of the "cube" with metal plate supports on the joints. If you just want to use it to support "typical" ritual elements, you might be able to get away with drywall which you can mud and sand to be seamless and smooth.
An idea to search for that may provide more technical information would be schematics for a wooden stage, or even a fun box (a skateboard jump ramp mated to a square platform) scaled down might work.
Power tools and saws will be important if you want precision cuts with the 2x4s and outside angles. A pyramid will require matching angle cuts on the inside of the outside boards to make clean edges, and will be much more involved; stay away from cheap pressed particle board as it will be difficult to get a smooth surface unless you plan to cover it with carpet or fabric.
Ritual items are very personal, and the connection you have with the item is more important than anything else in my experience.
I hope this helps you out; it certainly sounds like an undertaking.