How do I get a flat base....help

[mk1]

Ok, im going to hold my hands up here, Ive done a lot of work on my chassis and it is pretty much completely tacked together.....However I had a flood in my garage and didnt realise that to some degree this had warped the base board meaning it wasnt flat! Dumb...yes!

I have checked the dimensions and am not happy that it is accurate enough and realise that to spend more money on it i will need to take a backward step and rebuild it...after I have taken it to pieces.

Where I need the help is....how can I get a properly flat base? I have tried making a second one from timber but it is difficult to get timber also which is flat and checking it with an inclinometer shows this isnt quite right.....

Anyone who can lend some advice on this (or a flat base), your help would be very much appreciated.

Many thanks in advance,

Martyn