On my '87 CRX Si, I removed the headliner and the few bolts that hold in the entire assembly - panel stays attached to the motor/track assembly (it hurt a little when it fell on me, but that's another story). I then removed the panel, had it placed in the closed position, and then had straps welded between the panel and interior roof to hold it in place. You can then throw a bead of caulk around the exterior seal to make it a little more rainproof (nothing like having it rain inside the car during a wet race). As I don't think any Si of that vintage came without a sunroof, so I do not believe you can completely weld a piece of steel in place of the panel (isn't that just to replace glass panels?).
-noam