Yeah I should have taken more pictures during the construction process, next time I'll take more. I do have a couple though. I got this idea from bearded dragon forums, but I've seen people making aquarium backgrounds using the same method as well. I bought a sheet of 1" thick "Foamular" rigid foam insulation and a 25 pound bag of sanded grout in "Delorean Gray." I cut sections of foam into layers with each layer being a step for the turtles to climb up to an eventual flat peak, and used a utility knife to cut a slide from the peak down into the water.
I stabbed grill skewers through the layers of foam to keep everything in place, cut the ends off the skewers so nothing was poking out, and put it in the basking area to let the turtles walk around on it for a couple days, making adjustments where needed
I added the cave area because initially the turtles didn't have room to turn around next to the rock formation. I ended up making several adjustments based on watching them interact with it including ultimately scrapping one of the layers because it seemed uncomfortably tall once Franklin was up on top of it.
After making sure it was ergonomic enough for them, I rounded all the edges and carved details like cracks and other imperfections, and glued all the layers together with Gorilla wood glue. I used a lighter to melt all the flat surfaces of the rock formation to make them more uneven, then proceeded to glue a bunch of rocks to the structure, including covering the entire surface of the slide with the flattest rocks I could find.
Once all that dried, I started working with the grout. I thought I got the best results by mixing the grout very thin to where it would easily flow in between all the rocks on the slide surface, cover all flat surfaces and fill in all cracks, applying it with a plastic picnic spoon, using the spoon to guide it where it needs to go on flat surfaces.
The project cost about $40, but I still have 3/4 of the foam and grout left so I could make all kinds of other stuff for the basking area or even in the main tank.