little1337water and dish soap is your best bet in my opinion
True and make sure you spray from the top of your drop in otherwise you're just gonna get a puddle at the bottom which will slow you down. If you have any space under the bottom of your drop in, I'd definitely recommend drilling some holes so the water can drain and doesn't pool up.
Also mildly important: Keep the top of your drop in where you get your skis on wet!! If it's dry, your skis are going to be dry so then you drop in with dry skis and you lose out on some lubing until like half way down when the bases get wet. So keeping up there lubed will keep your skis lubed from the get-go and not only halfway down. It's not a shit ton of speed but if you need just a little more oomf, this could get you that.
I've also heard of setting your bindings back slightly if you have them at center because the friction holds your skis which a lot of people compensate for by not leaning so far into it. But obviously you still want your weight into it. So by setting them back you can get your body weight more into the drop in without going over the handle bars. So in theory you may actually have a bit more controlled momentum. I've never done this but it kind of makes sense? Maybe don't. I don't know.