Here's the jig I made. I used a scrap 6" long by 1 5/16" wide by 7/8" high. I beveled the top edges so the sled parts don't get glued to the jig. I put a stop-block on the front and a small finish nail on top. The finish nail can be adjusted to suit how far back on the runners you want the sled top.
To assemble, just put the runners so they rest on the jig sides and front of the runners rest on the stop-block. Put 3 or 4 drops of super glue on each runner. Hold the runners in place with one hand and place the top of the sled in place. The top of the sled should rest on the runners and be against the nail. (If the sled top rests on the jig, just sand a bit off the bottom of the jig.) Just hold the sled in place until the glue sets. I hold for about 30 seconds.
Here are some pictures. Hope this all makes sense.