Nite Ize has a variety of Tie Downs and Bungees that might be suitable.

The reason there isn't a bustling industry in devices like this is that for most kinds of line including paracord there are knots that work reasonably well for most time scales- e.g. variations on this https://en.wikipedia.org/wiki/Taut-line_hitch . For a gadget, simplest would be a line loc which is a plastic friction-only lock to replace a taut line hitch- see several examples here http://www.zpacks.com/accessories/spectra_cord.shtml. If this isn't permanent enough, there are also cam based gadgets that stand in for an old climbing ascent knot called a Prussik- there are several variations, but the Google-fu word they all have in common is "ascender"; see e.g. http://www.petzl.com/en/outdoor/ascenders . Hope one of these is some help!