I couldn't locate the thread either, so maybe its been deleted.
Alignment is quite easy, you need the apex of the rear prism to face towards the projector and the base of the front prism to face towards the screen. The angles of "displacement" will depend on the angles of the prisms used where the thicker the prisms, the less angle needed. I would attach an image, however all of my images have my logo on them and I am not permitted to display my logo as that is considered "advertising".
Your enclosure need only be wide enough to house the prisms once set at their angles (unless you intend to include a bi-pass). You may also need to have one side shorter then the other (exposed angled side of the front prism) to prevent vignetting. I'd suggest taking a look on the 235 screen thread (sticky in the main forum) for photos of the DIY prism lenses members have made for ideas.