Uwe wrote:

Could you explain a little more why a portable is too small to work? I don't understand that limitation of its mechanism.

Perhaps you've learned the secret in the erstwhile, but for others: Most of these desks had holes drilled in them to allow you to bolt the typewriter to the desk permanently. Then, when not in use, the top folds around hiding the typewriter inside of the desk and leaving the desktop flat and usable for other functions. As part of the design, the typewriter is then hiding inside the desktop upside down. 

Generally, standards were the only machines that had the bolt holes drilled into the bottom of their chassis to allow for this functionality. Because portables are small and easily moved, they really never had the hardware set up to attach them to these sorts of desks. As a result you could technically use them with such a desk, you just can't quickly flip the desktop over and hide the machine without damaging it. You'd have to put the machine away somewhere else and then flip the desktop over to have the traditional, flat desk.