90 deg countersink endmill
Another vote for the 90° cutter, which will be useful on other projects and it's not expensive.
Bolt the angle right to the table (no vise), with a piece of aluminum (or whatever) under it, spacing it up just enough that the cutter can't touch the mill table.
Me, I'd use a
45° wheel cutter like this. Typically intended for horizontal mills, but you can get an R8 arbor that fits them.
The main advantage is fast metal removal. If you have a long cut to make, this will take much less time than a typical little 1/2" shank chamfer cutter. The big cutter will stay cooler and last longer, and the fat arbor is way more rigid.
The cutter I linked to is under $30 incl. shipping, but the arbor will be expensive if you don't have one, and/or don't have another use for one.
I have two arbors for cutters like that:
The one in the forground is common, generic, not too spendy. I shortened mine to be able to bring it closer to the table but as you can see the cutter needs a spacer below it and that big nut. The one in the back is a Petersen Flush arbor, super cool but spendy. I lucked into mine used, so maybe start searching for one. This one can cut dovetails.
The cutter I linked to is for dovetails I think, so in order to cut the bevel you want, you'd flip it over, and run the mill in Reverse.
