Issues with thicken command

[Anonymous]

I have a piece that i need to add a compound to, 1 1/2 mm thick, all the way around. There is an issue with a radius in a corner, so I didn't use the thicken command there. I instead planned on just filling in that area using extrusions. Perhaps this isn't the correct way to go about it, but it was my strategy atleast. Instead of letting me do this, all I'm getting is errors. Does anyone have any suggestions?

Also, am I going about this the right way, I'm trying to make the "red" a seperate piece from the "gray".