Currently the plans for building this badass miniaturized replica of the Batmobile Tumbler from Batman Returns are for sale on eBay.
Basically you take your donor vehicle, looks like he used an ATV, weld a bunch of new framing and panels and viola, Batmobile and instant neighborhood stardom.
UPDATE – It looks like the auction has ended with a Buy It Now price of $34.99. I am sure you could contact the seller, tully711, to get a set of plans.
Here is the actual listing