How do I extrude a drawing 5mm, using the shape of the inside of a body?

[pruckelshaus]

I need to create a supporting plate for the base of a field element in the current VEX V5RC game.  Picture of the piece below, I have a STEP file for it.  The "cone" was manufactured with the walls too thin, and as a result, in use (robots banging against it), the mounting holes crack and fail.  My idea is to create a 5mm thick "plate" that goes inside the base so that when an impact does occur, the stress of the impact is shared across the base.

Here's what I have tried (with a basic understanding of Fusion 360):
* Import the shape (STEP file) of the base

* Sketch a circle the diameter of the base "under" the base on that plane

* Extrude -> 5mm (towards the interior of the base) -> Extent type "To Object" -> (Select cone as object) -> Operation "Cut" (but no objects show up in the objects to cut)

I get a "Cannot complete extrusion" error with message "Error: Cannot complete extrusion.
Cannot extend extrusion to object. The extrusion profile falls outside the boundary of the selected body. Select a face or plane instead, or adjust the profile so that it falls inside the boundary of the selected body."

And I'm stuck.  I literally just want to do a boolean join on the base, leaving the geometry for the inside of the base so that the two pieces would fit together, but I cannot for the life of me find a simple boolean tool for multiple shapes (when one exists in MS 3D Builder that works more or less fine). I'll use 3D builder if I have to, but I have additional operations to perform on the base support for a better 3D printed end product so would rather just get Fusion 360 to work.

Help? Please?

The base:

[jhackney1972]

Following your directions, the model is attached.

[pruckelshaus]

Only the original body and the sketch are there? Maybe I'm missing something?

[jhackney1972]

Model is complete as far as I can tell.  I will attach the model again.

[pruckelshaus]

I must have phrased my question inexpertly.  I was trying to extrude to a base "cap" like this so I can make some other changes in F360 in order to make the 3D print use less filament/print faster, not reinforce the entire piece...can't do that.  Cap goes down, cone shaped base goes on cap, attach to field.  If that isn't possible, I can make the grotty piece I did in 3D Builder work. Thank you.

[jhackney1972]

I am afraid you still are not making sense.  If all you desire is to reverse the direction of the extrusion on the model I sent, you can open the model and edit it.  If you want something different, you are going to have to explain it.  Your latest picture, shows nothing different than what I have already done.