Weight alone isn't usually enough to straighten a rekkid.

Try placing the vinyl on a heating mat, the type used by homebrewers to keep the beer at the correct temperature when fermenting. This is a low heat so won't damage the vinyl, and the combo of weight + heat seems to do the trick. Heating matts are ideal as they are just slightly larger than a 12" and flat and firm. Obviously use common sense and place some thin material either side of the vinyl to protect it.

I've also heard that yuu can get good results with the wedging between two panes of glass method. You need thick, heat resisatant glass. Clamp the record really tightly between the two then place somewhere warm, like an oven on very low heat or even in a sunny window.

Haven't tried the ironing technique, but a thick towel and an iron on low heat makes sense. Sounds plausible. I might try that on the next warped rekkid I get.