OK, I will explain what is running through my head in relation to both projects to facilitate the brainstorming process.
1. For the Sigil of Baphomet, I am thinking of getting either an unfinished wooden plaque from an art supply store, or an unfinished circular table.
I have access to an image enlarger and although it might be hard to line up precisely, I am fairly certain that I could probably pull off a suitable tracing in pencil and work from there, after applying a base coat of white acrylic. The rest I will fill in with black acrylic.
2. For the altar, I imagine that I will need to construct a framework first, and then simply nail thin wooden boards to this for the shape.
My biggest concern here is that the angles may be hard to achieve. Also this would have to be structurally sound if it were to support any weight. I could not possibly ask a female friend to lay across it only to have it collapse!
As for the Sigil, I think I can manage fairly well, but if anyone has made this sort of thing before I am very interested to know what kind of paint and varnish seems to work best.
For the altar, I am rather clueless as to it's construction. Even pointing the way toward a book that describes how to build similar things would help a lot. I am definitely good at geometry, but have never bothered about building a three dimensional trapezoid before! So any practical knowledge in this area will be very helpful to me.