Mark is correct - a copy of a field approval or even an STC is not going to cut it. You need to get the FSDO to stamp one for your airplane, or purchase an STC. In the olden days a field approval was enough to convince an inspector, but now when they see more than one request, they want you to do an STC. Opinion. And I am experienced; I got three field approvals in 2015 - took me almost a year of arm-twisting.