From my experience, a vacuum hold-down table won't work on a badly warped record; there isn't enough suction. May work for just a very small amount of warp, but a ring clamp (weight) should work better.
Those expensive record-flattening machines DO work, quite well. The catch is that they ramp up the temperature very slowly, over a period of at least 45 minutes, and then cool the vinyl off even more slowly, over at least two hours. You can set them to work even more slowly, which is even safer.
The temperature they reach is also not all that high; they are just warming up the record enough to soften it ever-so-slightly. Constant gentle pressure does the rest. V-e-r-y s-l-o-w cooling is critical, or else the warmed vinyl will just re-warp itself the minute the pressure is released. Low temperature is necessary to prevent "melting" of the groove details adversely affecting the sonics.
The reason people don't have much luck with DIY approaches is because either the temperature or the timing isn't correct. Usually they use too much heat over too short a time, and damage the groove information. It is hard to control things well in a DIY situation. The professional machines use laboratory-grade thermostatic controls to make sure the process is optimal, every time. That, plus the small niche market they are addressing, is responsible for the high prices they command.
You COULD DIY a good machine, but you need to incorporate precise temperature control with timing mechanisms. It would be a project at least as ambitious as a DIY record-cleaning machine, but not impossible.
I would make one, but I find very few warped records; not nearly enough to justify all the work it would involve.
There is one other problem: when the record is heated enough in the first place to warp it, often the grooves are already distorted from THAT heating/deformation, and no flattener will exactly undo the distortion that is already in the grooves. The greater the warp, the greater the distortion is likely to be. Really badly warped records cannot be fully restored, sonically speaking, even though they can be re-flattened. The machine will/may help reduce the groove distortion in some directions, by pushing things back more as they were, but it won't put things back exactly as they were, nor will it fully restore any small frequency information that is already "melted out". Whether you will hear the remaining bit of distortion is something you can only find out by playing the re-flattened record. So even when the machine successfully re-flattens the record, it may not have fully restored it.
The BEST solution is probably to just go find a new one that isn't warped. For minor warps, try a ring clamp, along with a cart/stylus that tracks well, on a longer tonearm.
