potentially make a filler piece , that can go in the tube as its squeezed in the vise,
and then you have 3 thicknesses of metal at that point the one you made + the tube wall thicknesses


made myself some rack struts on my last touring bike frame build . use 1/2" square tube.

bottle braze-ons for the rack , a tab of 1/8" steel for the frame end,
folded down one side of the square when I brazed it up,,

A rigid connection.. to my Bruce Gordon Racks..