Wouldn’t hurt to include a reference image rather than send ppl on a wild chase 
This, right?
I tried to create it using Render
Target
And you’re right, Render Target is not the way to go. You could simply attach meshes to the backpack using Attach Component To Component node. Or a combination of Attach Actor / Component.
Haven’t played the game so not sure how the inventory really works. You may need a grid system of sorts - again, not sure how it works.
Do include some details if you expect tangible help.