The constraints upon a Hound’s arrival in the present day out of the time stream are clear: it can only materialise through an angle of 120 degrees or less. An entirely curved environment — hiding in a hollow, Perspex sphere, for example — will prevent it from manifesting. Except, what if the hound materialises outside the sphere (via a corner)? Is there anything in the literature (game or otherwise) to say whether or not the Hound can then simply tear through the sphere using its physical strength? In other words, does the ‘no curves’ rule apply simply to the Hound’s materialisation, or is the creature stopped by curves, period?
Not for long, apparently
Running away, staying away from all angles, or fighting the Hound, are the known methods to escape. In the original novel, The Hounds of Tindalos, the creature is described as being able to move through corners of our Space-Time to feed on the blood of their target.
However, it appears that they cannot survive too long in our bound Space-Time and immediately disappear back out of our Space-Time once they have fed. The story is short, and we don't know how long the creature took to kill and feed on the victim (Halpin Chalmers), but it's assumed it may only travel through angles and it needs enough open space to be able to manifest itself once out of that angle.
The victim also claims that the smell they follow forces them to follow angles, and he also seemed to believe that if he could stay away from all angles long enough they couldn't "smell" the victim. So, if they cannot smell the victim, they will return frustrated to their home plane. We are not sure how much of this is fact or simply crazed babbling from his scared mind, though.
So, a small box shouldn't provide enough space for the creature to manifest, but the victim covered the angles of his staircase, corners of the rooms and windows, and any clefts with plaster. He also removed all furniture from his room. The creature manifested once a piece of the plaster broke and fell off after an earthquake, creating small cracks, which the Hound used to smell, manifest and then kill him.
The angles in his own body or clothes, or even in the writings on paper didn't seem to matter though.
Having read much of the entry, the real answer appears to be... whatever fits your story better. The point of the Hound is atmospheric - it's about being hunted by a creature that can find you in almost any place, at almost any time, that is relentless, and almost certainly beyond your power to defeat. If hiding in a plastic sphere gives your PC a degree of real comfort, then obviously that cannot be permitted, unless it is merely as a brief interlude of rest by which the return of bleak terror is made more stark.
To wring the most out of this experience, I would suggest that safety be achievable, but not sustainable - whatever that means for your story. It should be possible for the PC to manage brief spans where they are largely safe from the beast, but that each step along the way be consuming resources that they'll never be able to fully recover. If they do nothing but run, then eventually they will run out of time, or money, or will, or attention, or food, or water, or alertness, and that is when the beast will strike - like a hunter that hounds its prey over long distances until it collapses from exhaustion and can be easily consumed. If they turn and fight, their chances are slim. If they do nothing but run, they might eke out a bit more miserable time on this earth, but in the end they'll have no chance at all.