Seems like a good time for the
Acrylic to Glass section
For a pressure baffle in a sump, I would recommend using 3/8" material in lieu of 1/4". Also you can use just about any brand acrylic for this, extruded is game, as well as Lexan (polycarbonate) since you aren't actually bonding it. Another step I recommend is to do this using reinforcing strips that are siliconed flat to the glass on the downstream side of the baffle. Just a 1" wide strip of 1/4" material (again, any material, and it doesn't have to be top-to-bottom, length, but should go to the top of the baffle) and you can silicone this in prior to siliconing in the baffle or at the same time. The reinforcing strip, siliconed flat to the glass, will have much stronger long-term adhesion compared to a right-angle baffle attachment. This way you also don't have to worry about the edge of the baffle taking the load.
In the case of a glass baffle bonded in with silicone, the silicone between the end of the baffle and the glass is taking on the structural load, since there is a physical bond between the glass and the silicone. Since this bond doesn't exist between silicone and acrylic, the silicone "dam" on the downstream side (which is bonded to the tank glass only) is what takes on the load. In essence, it's only a silicone wedge, and since the acrylic baffle can bow under pressure, this can more easily fail. Adding the reinforcing strip will relieve the "duty" of the silicone, transferring the strength load to the strip which spreads out the load quite a bit.
This is really more critical for baffles with a decent level difference between baffles, or a lot of flow.
For overflow boxes (4-sided, open back)
@TaylorPilot's advices is pretty spot on. We've all seen the Reef-Ready tanks with an ABS plastic corner overflow, or custom glass tanks with an acrylic overflow siliconed in place. The reason this works is due to pressure along the full length of the box and the fact that the bottom of the overflow is sealed to the bottom of the tank, and also that there usually is a fair amount of water in the overflow tower.
However the thing to keep in mind with a 4-sided overflow is that there can be a level difference between the inside of the box and the tank water level. This creates a buoyant upward force on the box, and that puts a pulling tension along the bottom edge of the overflow box, so that can have a propensity to cause the box to "peel away" from the tank wall if you have a large (wide/long) overflow box, a deep box, or if you run it with a big level difference.
Now, this is not a horrible issue - it just needs to be one of those you are aware of. The second the seal at the bottom edge is broken, water will rush in and fill the box, and then the bulkhead/elbow inside will drain the tank down, worst case here is that your sump fills up more than intended, then if you have a power outage (or a poorly designed sump) you might have an overflow situation.
The fix, for peace of mind, is similar to the sump baffle fix, with one slight difference. Simply take a piece of 1/4" material that is about 3/4" or so wide and about the same length as the bottom panel (it doesn't have to run the full length, it can fall short of the corners by an inch or more) and solvent weld it to the bottom panel of the overflow box, perpendicular to it, flush with the back edge (the edge that will "touch" the glass). Now you have extended the surface area for the bottom/back edge of the overflow box. When you silicone the box in, instead of a 1/4" wide area of acrylic-to-glass, you now have a 1" wide area. This significantly reduces the chances that your overflow box will break away. The solvent weld doesn't have to be perfect in this case either, a rough-cut edge that is nice and straight, followed up by a few scrapes with a razor blade, and capillary welded in place (using the "preferred" capillary method) will work just fine.
