Attenuation boxes for a 100 watt Marshall are expensive, for several reasons. An attenuation box for a 1/2 watt amp should cost less than $10, way less if you build a fixed attenuator with 1 or 2 watt resistors. If you like the tone of your half watt amp, try slapping an "L pad" speaker attenuator between the amp and speaker.
I have an older "100 watt" L pad from Parts Express in one of my DIY "15 watt" guitar amps. It costs about $10 and turns the full metal racket into an amp that is usable in a concrete basement. My only complaint is that being wirewound there are small but noticeable discreet steps, and the first step above zero is too loud for headphones when the amp is at full crank which is closer to 25 watts than the 15 watts I measured at 3% THD when I built it.
https://www.parts-express.com/search?keywords=L pad&order=relevance:desc