It depends what kind of planes you are fielding there...there are numerous designs out there to accomodate most everything. A sheet of plywood works for startup, and /or any wedge that will deflect the exhaust upwards can be used I've seen them made out of plywood and metal......It just needs to be heavy, or well anchored so the blast does not blow it over. Some guys use lengths of stove pipe or dryer vent mounted on a length of 2x6 (or similar)

Here's a pic of one used at Jets over Whidbey...