Stacking power amps mono blocks

I have two tube mono blocks that are stackable. However when I put one on the other there is only 1/2" space for heat dissiation between the two of them. I am worried that the lower unit will fry over time if it gets to warm and the heat from the tubes can't escape. I can obviously put a 2" x 4" to create space. However, the look may not be great. Anyone that has figured out a nifty thing or purchased something that creates a nice 3 to 4 " gap between two pretty heavy (at least 50 lbs each) power amps. By the way if I put them into my Target stand (the ampsare 8 " tall (mono blocks) I need to remove one shelf and loose three shelves and not great look for putting in two amps. I am I just not getting it?