Putting a baffle in an existing sump is pretty tricky to get it perfect. If there is a level difference on each side, then it will need to be sealed around the edges. If there is no level difference, then sealing the edges is less important.
In general, you should cut the piece so it is as snug as possible, not loose. You need the acrylic joints to be tight for the bond to actually form. WO16 is not a very good choice for this, because while it is a gel-like product, it is not a gap filler - it will dry to the same profile as WO4. But you can build up layers of it, which will take time, and may still not work well.
What I would recommend is cutting the baffle so that it fits in the position you wish as tightly as possible. Then, use WO4 and the pins method to get the bottom edge welded on, let sit for 20 minutes, then turn the tank on one side and pin the joint as best as you can and weld that one, sit 20, then flip and pin/weld the third side. You could do this using capillary action, but capillary joints don't bite into each side of the joint well, and when you put water in the tank and the panels bow outward, those joints will eventually pop. So this rather poor application of the pins method is much better than capillary any day.
The result will be joints that are relatively strong (as strong as you need them to be) but very likely will not be water-tight. it is very hard to get water tight joints when putting a baffle in an existing tank/sump. So, now you are left with a couple options.
The easiest is to run a very small bead of silicone along each side of the joint. It doesn't have to be much - you are not attaching one panel to the other, you are just filling in the corner to seal it, so literally a bead the thickness of a toothpick is adequate, plus a thinner bead will cure faster (some would still say to wait a few weeks, but meh...)
Another option is to bond in gussets to seal up the joint. Cut a few strips of scrap acrylic that you have left over into strips that you can weld in to the corners on one side of the baffle, or both if you feel like it. Gussets need only be 1/4" on a side for this type of job, but bigger if you want. Use the WO16 for this, run a bead in the corner then place the gusset in there, press and hold. Also hold your nose. WO16 is stinky.
You might also just use the WO16 without gussests, and just build up layers.
Alternatively, if you can't get WO4, you could just bond the whole thing in place with WO16, but you will only be able to really fill in the bottom joint before you push it down, then the side joints you would probably have to gusset on both sides. You might be able to get some WO16 into a pinned joint, but definitely not all of it - it does not wick in, at all. If you only have WO16, gusset both sides of all joints and call it good. Your gussets should probably be bigger for this, because the gusset is actually holding the walls from bowing, so you have outward pressure on the joints, which are already somewhat weak from WO16 (which is really junk).
Clean all the surfaces with alcohol, rubbing alcohol is probably better than nothing, denatured alcohol is better.
Rough cut with a table saw, then scrape with a razor blade to get a nice smooth edge. Getting the piece square so that all 2 sides make full contact is going to be the hardest part.
Also Home Depot stuff is generally your cheap import extruded stuff, so err on the side of caution (i.e. take extra steps to make sure the bonds hold). Make sure it is actually acrylic and not polycarbonate, which won't work.
