I use fruit tree bird netting. I double it so that the openings in the mesh can be made smaller. It's in the Garden section in any home center. About $10 for a huge roll. Feather light. You can actually make a screen top with it with the material for repairing window screens. But that can cost a few $. I just wove thiin wooden dowels (that I had laying around for eons) through the mesh and attached it to the frame with velcro. hth
There was a thread where somebody used a netting that 's sold to retain loose-fill insulation, and put in a custom-made window screen frame top. I couldn't judge the size of the mesh, and couldn't find the stuff anywhere. I was also put off by the total cost after buying all the window frame componants. Not bad for a smaller tank, but I've got lots to cover. That's where I came across the bird netting. Doubling it isn't any big deal, the entire roll weighs ounces.
If you come up with something let us know. I couldn't find that insulation rentaining mesh anywhere. One thing to keep in mind is that the tree netting has UV inhibitors which I think would be beneficial if you keep it under mh lights.
There is a guy on Michiganreefers that has some that is 1/2in and possibly 1/4in that is clear mesh he ordered a roll and was selling some excess off I don't know if he has any left but you could do a search on there for clear mesh top it was in the diy forum the cool thing about the stuff he bought was that it didn't block any of the light. He made the type of top with the window screen frame and I thought he said the framing was actually pretty cheap but he documented it. I will try to find the post hopefully RC won't get mad if I link it but if it is a no no they can remove it.
Well, he did post what he had left in the for sale forum if I remember right. I remember the whole roll cost him in the $50 range. I was just a little turned off at the price for the whole thing when you add in the frames. For a 125 you would have to make 3 frames for it to be workable. Again, I need several, and this would have cost me over $100, and a lot of time piecing together those screen frames. For one single-opening tank like a 65G, it's definitely the way to go.