For a cracked Brother PS-300B industrial sewing machine case, the "solid piece" covering you likely need is the Top Cover Assembly or a specific
The Brother PS-300B is a programmable electronic pattern sewer. Depending on where your crack is located, one of these genuine Brother assemblies will provide a clean, "solid" fix: Top Cover Assembly (Light Gray) Part #LM4973001 Brother Ps-300b Crack
– This is the most common large plastic housing piece for the upper arm. Alternative Top Cover Part #SB0785001 For a cracked Brother PS-300B industrial sewing machine
If you cannot source a replacement piece immediately, community experts recommend these methods for a "solid" repair of the existing plastic: Plastic-Specific Epoxy Brother Ps-300b Crack