This Barn Door tracking platform is designed by Gary Seronik.  For alignment, I simply hand hold a polar alignment scope on the hinge pin. The first test, with two five minute exposures, showed nice tracking. The x & y misalignment of the images were 1.13 and 0.81 pixels. The rotational misalignment was 0.03 degrees. This was at a focal length of only 23 mm. It will be interesting to see how it works at 100 mm.

About the build cost...
If it is necessary to buy everything, you may want to consider purchasing an inexpensive equatorial mount. The cost of this tracker was a little over $200. I had to buy everything... even stain, varnish and paintbrushes. Living in rural AZ didn't help either. Several parts were purchased on-line, with ridiculously high shipping costs.