I know some people who put a layer of aluminium foil or copper foil glued to the wook, so it closes the cage, but I don't really know the difference in tube amps. I just asked this because I want to do something similar, but once I made an okko dominator pedal and got noticeable noise differences between open lid and closed lid hammond enclosure when I was testing it. Glad to know you didn't noticed any difference in noise levels.
BTW, I never heard of Ampmaker before, until about half month ago, when a guy at Handmades (brazilian diy forum) posted a picture of the SE-5A he just build. I did a quick search, and they have nice kits.