I have a couple of silicone log moulds that I used to use fairly regularly. Mine are the wiltshire brand, and they have a couple of little struts along the side to support the sides of the mould when it is full of batter. (soap or cake!!)
I have seen cheapie versions that don't have the struts, and they do have a tendency to bow at the sides when they get some liquid in them.
I know some have overcome this by placing the mould into a box or something that will offer some support.
You won't get straight regtangular or square bars with these moulds, more like trapezium shape. But they are a dream to unmould.