Kind of depends on how big it is and what you're doing with the piece after it comes out of the mold (which also determines whether you leave the bottom open or closed).
I think most optic molds tend to have around 15-25 degrees of included angle? (so the angle between each wall and the vertical centerline axis would be half that). I wouldn't go much lower than 5 deg, just so you have enough draft to get your glass out of it easily, nor much larger than 30 deg, or your piece will start getting mighty squat. That's from memory, so if someone measures their optic mold and I'm way off, please do holler about it and keep me honest.
Steel stock and welding gas are both cheap though, might be worth just quickly tacking up 2-3 different shapes and seeing what you like, then breaking down the ones you don't want to use back into scrap. Good on you for making your own tooling & molds, it's an excellent skill to develop.