Jerry, I say put the motel back! Just because they dug down under the boulder to HIDE doesn't mean they won't use the motel for laying eggs.
Turtles like hidey-holes they can dig into, especially when it's cool. Whenever the temperature dips a bit, one of my African sidenecks (a water turtle used to warmer weather) will dig into the dirt in a particular spot, and if I didn't know where to look, I'd never know he was there.
But hiding is different from incubating eggs. They'll still need a place to dig a round hole with their back legs, that is exposed to sun, and only about 5"-6" deep. Your turtle motel should be perfect.
The turtles know that under the boulder is NOT a place for the eggs to incubate, 'cause of lack of sun, plus, they're digging in front-end first. To lay eggs, they dig a hole with their back legs and put their business end down into the hole, leaving their head out to watch for predators. Laying can take a few hours. You've shortened the time by providing soft sand, instead of hard dirt. But the actual laying still takes some time.
My sideneck just laid 18 eggs. Sliders are larger and they can lay more! I think they lay later in the year... I'm going to Google and see if I can find when... I think it's mid to late summer.