The T-rig Insert is for Texas Rigging vs. the Tube Jig. The difference is the T-Rig has apox. 1/2" solid head and the Tube jig has aprox. 1/4" solid head so the jig fits all the way up. They screw into the spline so you can use either or both with the same mold. The spline will also work in all of our skirted molds.
The HD additive is awesome for tubes vs salt as it is much smaller and fluid. You can shoot them in regular and will get all the action, increased durability and it will fill the tail out really well. The key to the tail is to shoot it hot the first time to warm the spline. You could also use a hair dryer to warm the spline, or dip it in plastic, really anything to warm it.
I'm sure others will chime in as well.
Jason