I tried everything, but in the end, it was neither proseal or tape for me.
Using a big piece of angle will work, but I found that just drilling and clecoing directly to a flat worktop works just as well (and is a lot easier to drill through). I tried a version of pretty much every method mentioned on the forums, and at least for me, drilling/clecoing directly to the work surface ended up being the easiest way to keep the skins aligned and flat for drilling. I then pulled the trailing edge wedge out and used a 3D printed wedge holder and my drill press to countersink the holes. With everything prepared, I clecoed it all back together, but not to any work surface this time. With it clecoed, I just slid the whole trailing edge off of the bench and used a squeezer to partially set all of the rivets. Because the whole thing is clecoed while you're setting rivets this way, you don't really have to rely on any adhesive (I started using 3M tape, but by my last control surface I didn't use anything). Once the rivets were partially set, I flipped the whole thing over and used my large back rivet plate and a mushroom set to finish fully setting each rivet. I know some people fully squeeze the rivets using angled sets, but I prefer how a mushroom set can really finish tightening up the edge.
Had I known for my 9A that I needed to build a perfectly flat sacrificial worktop for my flap/aileron construction anyway, I would have built it early and used this method for all of my tail surfaces as well (the rudder might be the only exception since I think it would be hard to lay flat, but you could still figure out how to get it done). With clecos in place the whole time, you don't have to worry about proseal or tape at all. My flaps were my last control surface and where I finally figured out the method that worked for me. The trailing edges have zero wave, and this method was by far the least stressful and easiest for me.
Whatever you try, don't stress too much over it. The first few times are going to be a learning curve no matter what. My trailing edges got exponentially better with each one.
(sorry for the goofy photos - no matter what I try they rotate on me)