Thinking out loud here...
I noticed a few companies offer expensive and not thoroughly engineered 4 wheel conversion kits for the polaris slingshot.
I was thinking if you started with a used slingshot and put in a bit of effort, you could have a light, fairly low cost tube frame car, with minimal time and labor compared to a full build. The front of the car seems fairly well done, suspension wise.
My first thought was to lift the rear diff, halfshafts, uprights, suspension arms, etc out of a pontiac solstice, weld up a rear subframe, scrap the polaris driveshaft and belt drive converter box, toss in a longer driveshaft, and voila - awesome car. Stock slingshot is supposed to be 1750ish lbs so figure you could get it under 2000 easy.
I am going to attach some photos below to get the juices flowing. Feel free to add any comments.
stock slingshot
slingshot chassis
slingshot stock drivetrain
solstice rear suspension
other companies attempts at 4 wheel conversions: