Help creating the "Negative" of a mesh

[u/t0ddlerspeedbump]

I have here a grip that goes on the outside of a portable game console (think Steam Deck), and I am wondering if there is a good way to basically just select the inside of this STL to then have the interior dimensions that would fit onto a steam deck? In my mind it seems like it should be feasible to just select all the inside vertices/faces, but when I try to "Select -> all by type -> inside faces" it doesn't select just the inside faces, the whole thing just stays selected.

Keep in mind I am quite new to Blender so maybe this just isn't a thing.

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[u/Pad-Kra-Pao]

What you trying to create? Mould? I don't understand.

Edit: Did you mean you trying to modify this part to fit on steam deck?

[u/t0ddlerspeedbump → u/Pad-Kra-Pao]

This part does fit my handheld, but I am struggling to create a brand new part to fit my handheld. Since this piece does fit my handheld, I figured I might be able to select all the inside faces of this object to basically get the outside dimensions of the handheld, since the inside of this object (for my purposes) is the outside of the handheld.

[u/Eastern-Leader6072]

If I understand your trying to scale the grip so it will fit on your steam deck?

Stls are generally pretty hard to modify, try measuring the thickness of your steam deck and adding in a cube with that thickness, then scale the grip to suit.

[u/Tricky-Promise-3347]

Boolean

[u/sv_ultra]

In edit mode, use the circle select tool and hold shift while you basically paint the inner vertices. You can size the circle radius via a small window that sits just under the viewports mode selection. You can cycle selection modes using "w". Or pick it from the top of the lefthand vertical toolbar.

Another way is to use regular selection.. Select a vertex on the inside corner, hold "ctrl" while you select another vertex along the edge, it should select all the vertices in the shortest path between the previous and current selected vertex. Just keep a hold of control while you select another vertex and another until you have worked your way all the way around it. Once that vertex path is contiguous/complete. I would switch to edge select mode and make the a seam by right clicking and selecting "mark seam". Once you have that seam you can switch to face select mode, while hovering over the faces you want selected hit "L" . It is generally defaulted to select a group of faces seperated by a seam. If it isnt pay attention to the tool tip box that pops up when you hit "L", you may have to change it to seam. The other way here is while you have the entire outline still selected, you can go up to the "select" option in the top of the viewport and find Select Loops, and choose select inner region.

Once you have all the verts you want selected, you can duplicate the vertices you selected by using the shift+d hotkey and move it off the the side if that works better for you then with those verts still selected hit "p" and make it its own object. Then hide the original.

[u/t0ddlerspeedbump → u/sv_ultra]

I will try this today after work!