That's actually a piece of clear perspex I found in amongst the stuff in my dad's workshop, it was lucky that it happened to be just wide enough to fit, all I had to do was cut it to length and drill some holes for the mounting screws, and then fit some suitable pieces of wood scrap to the cabinet.
Part of the fun of making home-made guitar amps is searching for suitable material to build the amp, most of the bits and pieces I scrounged while working in my dad's workshop, I did have to buy one expensive item though, which is the output transformer, if my memory serves me right I think it cost me something like $380.00, or thereabouts, and then when it came to the power transformer I didn't know where I could buy a suitable one, so I found a power transformer that came out of an old reel-to-reel tape recorder, it looked like it was about the right size, so I proceeded to strip it right down to the cardboard bobbin, next I measured the dimensions of the transformer's core, and then did some calculations using a book called "Build Your Own Coils And Transformers", and then re-wound both the primary and secondary windings of the transformer according to my calculations, the transformer actually worked first go.
This time round, I'm going to go ahead and buy a proper, commercially made power transformer.