Making a sky dome?

For a project I am doing, I am trying to make a skybox that has a post-apocalyptic feel to it. For this, I wanted to make a sky dome, but the problem I am running into is that the mega prims won't let me walk in the hallow part of the sphere. While I could do a flat box, I would rather not as it doesn't look good to me, is there a way to do this with as few prims as possible? I have read that sculpties may be a good work-around, but I have no idea how to make them with what I already have on my computer. Any suggestions?

Most of the giant spherical sky structures around seem to be phantom. Sure, that means you will plunge to your demise if you stumble into the wall, but that fits in pretty well with a post-apocalyptic theme, doesn't it?

Doh, forgot about the phantom setting. Thank you!

What I would do personally is make the giant sphere phantom but line it with invisible normal-prim walls. You won't get the exact shape but oh well.. and don't need to do the whole thing, just the ground level as if you're flying you cant just fall anyway