It is possible to erect Shields inside one's mind, in addition to the more usual defenses around it. This is a somewhat specialized technique, and probably only of limited use. Quite possibly the most obvious application is to keep one's most private memories private when mind-linking; while the most common is the subconscious repression of traumatic memories, as an internal defense mechanism...but the latter wanders into the field of psychiatry, and is well beyond the scope of an article of this kind.
A nice basic internal Shield is a slightly elastic Mirror configuration, with a smooth, glossy surface that bounces would-be intrusions harmlessly off its exterior. Such a Shield would dent inward a little if pressed, but immediately resume its normal form once the intruding thought-form was sent on its way.
Such internal Shields are constructed in the same manner as a regular Shield, and, in theory, any Shield you can construct normally you can make internally, too. In practice, however, the variety of internal Shields is more limited. This is the inside of your mind we're talking about here. In fact, some kinds would be physically inside your skull. You'll feel it any time the Shields touch anything (with a couple of odd exceptions), so you'll want something that's comfortable, right? Heavy-duty, strike-back-on-contact combat Shields could cause problems.
It may be wise to set a time limit when constructing an interior Shield, rather than simply wait until you want to drop it, to deal with dispelling it. Or, better yet, set it so that it will remain for a certain amount of time or until you choose to dispel it, whichever comes first. Otherwise, it is too easy to mess up the setting and have your own thoughts bouncing off it, thus unable to drop the thing, and that would not be good. (Can you tell that I speak from experience here? No need for you to repeat my error....)
A similar sort of block can be produced hypnotically, either in one's self or in another person. I tried a self-induced one out of curiosity, and it formed the sort of default-pattern Shield I mentioned earlier.
Dr. G.H. Estabrooks, in his "Hypnosis Comes of Age"*, mentions the use of a similar-sounding block in 'training' a kind of mind-controlled spy-courier in World War I. He took it a step further, and locked the blocking in place to prevent interference with, or even knowledge of, the block by anyone other than the courier's two handlers....specifically including the courier himself. Whether the block/shield produced no sensation in itself, or whether the courier had been hypnotically commanded not to feel it was not indicated in the article. The courier had been programmed to forget that he'd ever been hypnotized, and he had no idea that he was working as a spy, so it could have gone either way.
In fact, I am only guessing at the similarity in structure between the blocking Dr. Estabrooks' methods produced, and those under discussion here, based on my own experience (self-induced and otherwise) and descriptions of similar phenomena by other people who have experienced them. In the case of such descriptions of blocked memories, obviously the block eventually failed.**
It happens sometimes that blocking, of whatever nature, cracks or shatters after a period of months or years. If you have cause to construct such a block in your own mind ~ or someone else's, for that matter ~ you may want to bear that in mind.
* Science Digest, April 1971, pp 44-50. It can be read on the Internet at www.mindspring.com/txporter/scidig.htm
** For some examples of such published accounts, see pp 28-31 of "The Controllers" by Martin Cannon