Maybe try a miniature version of this.
rectangular rock/tile - easy to break with precision. absorbs heat well
acrylic - easy to score & snap with acrylic knife. keep your turtles from escaping.
egg crate - all around easy to work with. ramp from water to platform.
Use marine grade adhesives to attach materials.
I made this over a year ago and I only now feel comfortable sharing because I have since upgraded the housing for these 2.
I believe the 70 gallons was inadequate.

