I’ve had my macbook mounted under my desk for a while. I original had it on some U-shaped aluminium extrusion brackets. If I was doing something taxing it would overheat a little, so I drilled some holes where the fan is, but that didn’t really make that much difference. I was looking for something else, and eventually decided on a cheap IKEA wooden shelf mounted with u-shaped brackets. I just made my own out of the same aluminium I used before, but I’m sure there are other brackets around like these robot servo brackets.
Here are some pics of the original mount, new shelf, and all the other things I have mounted under my desk: