Go down to your recycling centre and grab 20 cardboard boxes. Flatten them out properly. Laminate 2/3 them with a pot of adhesive and you have a slab of 2ft x 3ft.
Cut a slightly oversized outline of your bass out and line it with an old T shirt or three, with a backplate
The rest of the boxes make a prop that strategically slots onto your outline so it can lean back.
Spray bomb it matt black and it will look like a bought one.
You'll figure it out.