RHINO/RHINOCAM: need to add thickness to a 2D NURB while keeping face count low

This one’s a bit of a doozy, and is going to involve a LOT of explanation, so bear with me. I’m working on a project that involves .objs from a video game (weapons from Halo 5, to be precise), splitting them in half, and CnC milling them into positives for a mold using RhinoCam. The issue i’m running into is that i need these 2D meshes (turned into NURBs) to have a 0.25 inch thickness, but there’s no function in Rhino that allows me to offset the surface without mangling the geometry on the “outside” of the model, and simply copying and pasting the model, then scaling the copy down by a small amount, then combining them doesn’t work since the model is so complex. I got something close to what i wanted with a function in Maya, but it creates such a dense model that by the time i get to toolpathing any function freezes the program for upwards of an hour before it manages to complete it. so hopefully I already tried OffsetSRF and like i said, it absolutely mangled the geometry. I can’t just make both halves solid along the split line, because that would defeat the purpose of the mold, which is to have a two part mold shell that’s fitted together, and then have expanding foam fill the middle void, giving it strength while also staying lightweight. I’m very new to Rhino, so the solution is probably VERY far outside my current skill range. Hopefully i’m not asking for the impossible here.

Here’s some screenshots of the layout, which should hopefully illustrate what i’m talking about a bit better:

Eh offsetting a mesh from a game item…ugh that sounds unlikely to work, yes. I mean offsetSrf can handle some more complex situations, but you would need a clean NURBS model to start with to attempt that. I guess the question would be, if the goal is to mill a mold for…vacuum forming I guess?..why does the model need to be thickened?

Not sure, but what if you just programmed your CNC toolpaths to leave a stock of 0.25" on the part when finished?

Good morning @ryanhoult

I do things like this pretty frequently. But I take a different approach. In that I would model a box around your part like in the very simplified image below. You can also add keys to each side so the two halves nest into each other properly. You can then put a hole in one side of it to pour your foam or insert you nozzle from a metering system.
Sorry I don’t have time to show you a real example.

Hope this gives you an idea or two.