How do I draw this in 3d?

[Reinoud]

Hi all, I'm a noob when it comes to 3D printing. I'm a mechanic and I want to design some very specific tools to make certain jobs easier and more consistent. We have a 3d printer at work and I want to start designing my own little contraptions so they do exactly what I want. The people responsible for the 3D printer aren't very experienced and I basically want to show up with a Usb-stick asking "could you just print this file for me?". Nothing fancy at all, just really basic parts. At the moment I'm trying to create a guide to help me cut plastic glue tips to a certain shape. I have done some 3d drawing in Sketchup in the past, but nothing overly complicated. I've also tried Tinkercad, Freecad and Fusion 360 but I just can't figure out how to create the object I have in my head.

In the pictures you can see the original part, the second picture is similar to the wanted result.

I want to create some sort of sleeve where I can insert the uncut glue tip, which is basically a cone. I want to have a guide to make the V-shaped cut. I need a surface to lay my knife against and make a controlled cut, holding the knife at a 45° angle.So far I managed to draw the original tip in 3D but I can't find out how to create the surfaces at the 45° angle onto the conical shape at the exact position I need. I'm pretty stuck at the moment and in need of some input from people who are better at this

Any ideas on how I should approach this design? Thanks for the help.