In making the latest pulk, I was able to figure out the pulk to pull bar connection. It involved attaching a foot long length of metal tubing to the metal tubing of the bike trailer. This metal tubing has just a slightly larger inside diameter than the outside diameter of the pull bar. You can see it on the lower right corner of this photo.
The tubing was drilled out at the mid point and then towards the back. These holes allowed bolts to be threaded through it and the bike tubing (also drilled out by me). Locking washers help keep everything tight and together. A closer look at the connection can be seen below.
The small hole perpendicular to the ones holding the bolts was drilled out to fit a locking pin. This pin runs through the metal tubing and through holes in the ends of the pull bars. You can see the pull bars and the locking pins keeping them in place in the photo below.
The locking pin is attached to the metal tubing using a strap so it doesn't get lost. I put a grommet at one end of the strap and attached it to the tubing using a screw.
I also added a little bit of art work to the wheel saddle for a little extra flare.
So there it is. I spent less than $30 on each pulk. Things that might add to your cost is not having old skis laying around to use or not having a bike trailer. Old skis can easily be found for cheap at garage sales, thrift stores and on craigslist for less than $50. Old bike trailers can be found the same way. I don't think it is unreasonable to be able to do this project for less than $150 starting from scratch.
It's been a pretty fun project and I hope it helps people get their kiddos out in the winter time. Getting outside makes winter so much more enjoyable (and seemingly short).