How much do you want to spend?
At the low end of the scale, there's a spaceframe chassis made by Rorty as a direct replacement for the Beetle. You can buy the plans from them. Pm me if you're interested & I can send you some info.
Higher-up the scale, I'm using a modified Lotus Europa chassis under mine. It was done by Spyder Engineering for about £2,600 (bare chassis only) iirc: