Stretch resistant rope. Polyurethane is commonly used.

Tie an overhand loop in one end and from underneath, start threading from back to front.
Ideally you end up with a diagonal pull.
When finished, run the tag end through the loop at the beginning.
Yank on the tag end to tighten, half hitch it, tie it off.
A trucker's hitch would be another knot that would work.

Instead of the knots you could form a second loop and integrate a ratchet from a ratchet strap.

Edit:
I've never seen it done, but I imagine that a 1" ratchet strap system in its entirety could threaded through those 1/2" holes and voila! you have a ratchet standing by underneath to pick up the tag end.
Depending on the strap material in might sag when wet, be it would be easy to tighten.
This could turn out pretty sweet.