It's egg crate. Had a scan through and looked at your ramp (one of my early ideas was to make a very similar ramp!). It's probably about the same width and angle, maybe just a little bit shorter. A couple old pics here:
viewtopic.php?f=6&t=15260But instead of being centred, like yours looks like it is, it's off to one side. Frankie's not the brightest bulb in the box, so I left just enough room for her to swim with her head up between the ramp and the side of the tank, in case she felt she got "trapped" on one side of the tank.
Centring (and widening) the ramp might help, as it seems to be when she goes down with just half her body on the ramp that causes the biggest issues.
The other thing is, well, Frankie is a bit of a heifer. She's only about 9" long, but she's almost 4" "tall". I've never actually weighed her, but I'd guess she's 1.5-2lbs. So she gains a lot of momentum in the ~3" between the edge of the platform and the water. Then she sinks like a stone. Seriously. If you were to drop her and a similarly sized rock in the water, they'd probably reach the bottom at the same time. My boy Ralph, at 7" long is a real lightweight. I've seen him dive off the basking platform and he never even get's halfway down the tank.
Anyhoo, I'm rambling

So anyhow, when Frankie chooses to do a "controlled decent", she just grabs onto the holes in the egg crate and crawls her way down. But if she tucks her legs in, she becomes a projectile. I wonder if acrylic is a bit less slippy than egg crate?