I will surely be relinquishing the Rube Goldberg Complexity Award for writing this, but really I do not see a need for a hinged or articulated "shoe" on a tool for THIS kind of use. Basically the tool is only to guarantee you drill through the apex (tangent line) of the tube under the gusset. It's all on the same axis, because you are drilling the same tube you are using as a guide. Unless you have a curved tube, it's all in one straight line. A v-block with a tongue like my sketch would do that acceptably, while being the easiest to fabricate, machine, print, etc. To me, the most important thing by far is that with the simpler version of the tool, the same amount of tool manufacturing/machining/printing can yield three or four tools (of different hole sizes or rivet spacings) in the same time as one complicated articulating tool.