Since it looks like your handy you should be able to make one quite easily, bend a pc of 1/2" round rod into a C, weld a tube on the chassis that it can slide into and mount a spring to hold the tension, you can either make a wheel out of UHMW or buy one, here;s an example of a simple chain tensioner
http://www.hyperracing.com/product.asp?prodid=08-600