It sounds like exactly what I would have done (and have done in the past). I HATE the reactions I get, but the safety of my child is more important than what they think of me.
Once I got really upset because I was visiting family and I had asked them to provide a car seat for DS, which they were more than happy to do. I gave them his weight and height measurements. He was only 2 and they turned up at the airport with a really dodgy seat with the foam all broken for an old child (just a simple booster seat). I told them I would buy one if they couldn't get one, but they insisted. I was so angry, I went down to the shops and bought a new one and although they never said anything I know exactly what they were thinking, but I didn't care.