The 24-month pitot/static check performed by an avionics shop doesn't ensure an accurate IAS under dynamic conditions. Depending on how it's performed, it really only ensures that the altimeter performs with 43-13 tolerance and that your transponder encoder is providing accurate output. Flight testing a pitot static system is a challenge in any airplane, and every RV is different so accurate displays require individual testing of each plane. If you look in the flight test section, Kevin Horton has a great thread that will have links taking you to his web site. As has been pointed out, the Van's approved solution of the modified pop rivet has proven to work well; but it sounds like you've been down that road.
Here's a summary of the basic process: build or borrow a manometer and check the accuracy of your airspeed indicator. Then obtain GPS data and enter that along with your manometer data and the altimeter error from your 24- month check (or altimeter data card if it is a certified type), and then determine the magnitude of your "static source pressure error," i.e., how well your static ports are working. Mr. Horton wrote a helpful spreadsheet that does the math for you and will show the amount of airspeed and altimeter error present in your system under dynamic conditions. After you have this data, then you can start making changes to the port configuration. You can even approach this incrementally by using tape in front (if your IAS reads LOW) or behind the port (if your IAS reads HIGH). A variation is to cut adesive backed paper hole re-inforcements in half (the kind you get an office supply store) and affix them around your port--this will look like a 1/2 washer. If your current port is flush, this is fairly straight forward. On our plane, each layer produced a nominal .45 MPH bias in IAS. You can approximate a magnitude by looking at the difference between CAS and IAS you determined in your initial test, add layers incrementally and re-test. This technique allows you to "tune" the static system to mitigate error. Since you've already got a half washer in front, it may just be a matter of adjusting the thickness or removing it all together.
Now, having said all that, keep in mind it's not all that important (from an IAS standpoint), since your cross country performance is mearly a function of ground speed and fuel burn, and the airspeed indicator is just an approximation of angle of attack ("lift reserve" if you will). Until we developed GPS and advanced cockpit displays, I would wager most error went undetected (at least for non-certified airplanes).
On the other hand, if you have advanced cockpit displays, TAS or winds aloft data are only as good as the input; so if there are air data errors (e.g., static source pressure error), there may be errors in the displayed output. Depending on the system it may be possible to enter a correction factor vs. changing the configuration of your static source.
Hope this is helpful,
Vac