conceal water pipe - interior room

I need to re-route about 25 feet of pipe (either 1/2" copper or PEX) to the interior of a finished room (already drywalled and painted white) where the wall meets the ceiling, snug into the corner along the length of the room.

After it is installed, I'm looking for an alternative to boxing in with plywood or MDF board, then drywalling to cover the plumbing.

Someone told me he had seen a specialty building product in a U-shape, and thought it may be obtained in an L-shape. Something lightweight & rigid.

I called a building supply and a plumbing supply place, but no luck.

Any ideas?

An alternative might be to nail in a length of molding which is already cut at a 45 angles both back edges.

There is nothing made for this specific purpose, so you will have to improvise. Your idea of crown molding might be the best looking choice. If you have to make a corner, it's a tricky business to make the corner cut. I would never use MDF, partly because it is filthy stuff to work with and partly because it will not hold nails, screws, and/or glue worth a darn.

I defer to Gary but will share that I have had excellent experience with MDF - especially in crown applications.

With crown moulding, yr working with 2 planes that might me slightly out of level. The MDF does a great job of bending to conform to the surface. I have found it extremely nice to cut (no splintering; easy to cope and miter) and I have found that it holds nail and glue very well - just as well as wood. I would, however, make sure you do BOTH. You can use crowned staples instead of finish nails for improved holding pwr too.

You should of course support the pipe FIRST and independently to the studs. Then attach the crown just to conceal.

If you don't have enough room in the cavity of the crown, you can slice back the face of the drywall and run the pipe in the cavity.

To that end, it might be simpler to run the pipe at chair-rail height. Cut out a 1/2" notch in the drywall , drill the studs, then run the copper/pex thru the studs. Cover the gap with chair rail. This'll only work if you have no windows on yr run.

I should have been more clear. I have a personal dislike of MDF. I tried it twice. The first time I ruined a router bit, the second time it would not hold the cabinet door hinges on a cabinet. In addition, it is filthy stuff to saw. I suppose it has its place, but I prefer wood.

I hear you. Duly noted on the cabinet door application. I won't try that. Yeah, it routes like c*@p. IMHO, it's really best for molding - painted molding.
It creates a lot of dust when it cuts, but the cuts are buttery smooth which makes scarf and miter joints a PLEASURE.