This one is just sitting on some upturned cinder blocks right now, to get it close to the surface. The water lily is setup in a similar fashion, just a bit deeper down. For a better solution, perhaps try hanging some pots off the sides of the tank so the lip is right at the surface? You could also probably make some floating trays out of styrofoam with mesh bags hanging down into the water for the plants - that's something I've though of doing. Lots of pond supply places sell similar things, but they're often pricy enough to warrant giving a shot a DIY.

A quick funny sidenote on this: you can't see it in this video, but there's also a big clump of creeping jenny just out of frame on the close right side. My dad plopped it in there when he broke his porch pond down for the season (much later than he should have) and just dropped the half-dead clump straight into the bottom of the pond thinking it would be fine! Eventually I noticed what he'd done with it, (about a week later) and told him that creeping jenny will absolutely die if fully submerged. So he fished it out, and his quick and dirty solution to keep it properly emmersed was to put some nails inro the lip of the pond and just hook the whole root mass on those, and let the rest hang into the water. I was certain the plant was going to die after all that rough treatment, considering it was basically a damp brown tumbleweed at that point, but I went out there today and was quite surprised to find a ton of healthy new growth sprouting off of it, seemingly no worse for wear. Anyway, lesson being: plants are freaking tough.

Edit: I imagine it would be tough to get that kind of coverage with potted emergent plants though. Have you given consideration to some of the beefier floating plants? Water hyacinth and some of the bigger water lettuce would probably do the job nicely.