Fitting items into a container to achieve optimal efficiency is a science into itself. Ask FedEx, UPS, DHL, etc. I would think stiff construction paper and a pair of scissors would be the simplest way.
LibreCAD is strictly 2-D only. Same with the paper and scissors method. If this fits your purpose , in LibreCAD you can easily move around shapes within a rectangle (box). But probably you also will have to consider the height of the items and the box. So apart from the ground view you also have to draw at least one side view. Or use a 3-D cad software, which usually is much more complicated. FreeCAD is the big opensource program for this, but very difficult. You may have a look at SolveSpace, small, but maybe still too difficult.
For me this sounds like nesting for laser or water jet cutting or CNC routing.
Generally this is possible with LibreCAD, but presumably not very comfortable.
Using blocks for the items may be useful and simplify the work flow.