Use two clean 330 x 330 mm ceramic floor tiles, each with a smooth top side.
Preheat oven to 120-160 degrees Fahrenheit (say 65C).
Place both tiles in the oven, face up, on separate shelves so they heat through thoroughly.
Leave to heat until the thermostat is steady at 65C.
Using protective mitten, remove the bottom tile first and place face up on a heat resistant surface.
Now place the record onto the tile (in a clean paper inner sleeve) and put the second tile face down on top. The smooth side of each tile should be touching the record. The weight of the top tile will be sufficient.
Leave until completely cool – at least an hour.
On removing the record, it will be flat, and undamaged.