After my daughter was born, I did a little baby-seat mod so we could fly around with the little one. Basically, I added 2 beefy angles to the rear spar carry-through, then bolted anchor points (courtesy of the Home Depot aircraft supply aisle) to the angles. After that, I added some steel anchors to the existing seatbelt anchors that would allow a tie-down strap to slip through. (pictures at the link below make that easier to understand). The end result was the ability to very securely tie down a baby-seat base (2-piece type) in the baggage compartment. Probably over-built, in fact, since it secures way better than it does in my car.
Since I wanted it to be rear-facing (like in a car), that left little head or leg room for the kiddo. While she was very little (less than 6 months), she fit just fine, though. Now she's 7 months and has outgrown it in this configuration. We're about to upgrade her baby seat to a forward-facing type, so I'm going to see how well that fits when we get it.