If he can maintain concentration for a while, how about if they throw a Major Image specifically at creating an illusion of a clear glass / clear quartz prism, the size and shape of Keltham's rougher image of a prism.

If he can maintain concentration for a while, how about if they throw a Major Image specifically at creating an illusion of a clear glass / clear quartz prism, the same size and shape of Keltham's rougher Silent Image of a prism.