Mmathis,
Would your turtles be able to climb a totally vertical wall of hardware cloth? If you don't think they would attempt it and fall over backwards in the pond, then I might have something unique to entertain your thoughts.
Build a long, shallow, rectagular box to serve as the base of the barrier wall and also as a planter box. This would rest atop your false bottom and also be semi-buried throughout the terrestrial area of the pond.
Inside the box, it would be filled with a good mix of garden soil, sand, peat moss, topsoil, etc. Then, up from the center of the box and throughout the length of the box, construct a girder system that you can hang hardware cloth upon to serve as the turtle barrier. Use some sort of hardware cloth that has large enough openings for plants, but not so big that the turtles could get their feet or heads stuck in it.
Then, in the soil within the box, on each side of the barrier hardware cloth, plant some sort of ivy or vining plant, something that likes to climb and would thrive in your wet environment. Once it was established, the barrier wall would be hidden by the vegatation and look very natural and aesthetic.
Of course the choice of the ivy or vine would have to be non-toxic and hopefully perennial or maybe just a fast growing annual or biennial, but not something that would overcome the rest of your pond.
However, I also like Adavisus' idea of the bamboo wall...
That could be made to look just as intriguing without as much maintenance or fabrication requirements. But, it would also serve its purpose and look a lot better than a lumber wall or a plexiglas wall.
Otherwise, how about this...
Street pavers! Red, street bricks stacked to make a vertical wall and stuck together with, rather than concrete or mortar, Loctite® PL® Polyurethane Concrete Crack & Masonry Sealant. (Not recommended for continual submersion in water, but you can probably work around that enough by the wall design).
You could stagger the bricks to allow some small openings that the turtles won't normally peer through to the other side (higher up on the wall) but will allow air flow and light to pass through in a few key spots. For underwater areas, you could leave just a small slit between bricks so that surface water flows freely, but even the thinnest turtle cannot fit into, not even sideways.
Lot's of decorative ideas that you might conjure up that will also be effective and inexpensive (or at least reasonable).
Gordy