diff --git a/build.rs b/build.rs
index 39e2b47..78a951d 100644
--- a/build.rs
+++ b/build.rs
@@ -205,12 +205,13 @@ fn write_sim_data() {
     let segments = [(StreamType::IMU, motion_output), (StreamType::GPS, gps_output), (StreamType::Annotations, annotation_output)];
 
     // Write out the stream index header
-    rmp::encode::write_array_len(&mut unified_fd, segments.len() as u32).unwrap();
+    StreamIndex { count: segments.len() }.write_rmp(&mut unified_fd).unwrap();
 
     // Then the streams
     for (stream_type, stream_path) in segments {
         let mut fd = File::open(stream_path).unwrap();
-        rmp::encode::write_ext_meta(&mut unified_fd, fd.metadata().unwrap().len() as u32, stream_type.into()).unwrap();
+        // FIXME: Replace this with the actual rmp types in simdata
+        StreamHeader { id: stream_type, size: fd.metadata().unwrap().len() as usize }.write_rmp(&mut unified_fd).unwrap();
         let mut buf = Vec::new();
         fd.read_to_end(&mut buf).unwrap();
         unified_fd.write_all(buf.as_slice()).unwrap();