I have built several racks over the years.
Several of the first ones were "removable" type racks that had free standing legs. These were secured to the truck with tie downs to the bed ancors. While they worked well, they all suffered from the stability standpoint. They work against the truck and over the years (expecially in rough country) they begin to squeek and become more unstable in the process.
The last rack I built was not from the ground up, but more from the bed rails up. I still wanted it to be removable, but alot more permanent if needed. I looked at it as a long term deal, and started off with heavy built bed rails running all the way to the tailgate. I then built the "base" of the stand to sit on top of this very sturdy platform. I used the same large angle iron as the bed rails as the base and built off of that. When doing this remember that all beds arent "sqaure" and you need to measure accordingly. I drilled holes through the bed rails and put sturdy bolts through the "base" and the bed rails to secure it(both from the top, and the inside of the rails). I used long strips of a round hay bail "bailer" rubber rollers as a buffer between the bed rails and the "base"(got it at tractors supply). Might not be the best way, but I used gorilla glue to secure the "strips". There are tons of ways to go from there, but I feel the base is what adds the support, you really cant go wrong if you get the base right. I have taken tons of ideas I have seen over the years on other racks and formed it into what works best for us. With all the pics on this sight it wont be hard for you to do the same.
As long as it turns out as a sturdy shooting rest and doesnt make too much noise you have done a good job.
Take care,
Todd