I did something similar but it didn't turn out half as nice as serials..I was too impatient and I was applying black substrate to a white piece of PVC..lol This piece ended up in the trash after 6 hours of work on it. -.-
If you apply the silicone and do it in sections you seem to get better results..most of my stuff fell off cause the surface of the silicon dried too much before I coated it.
You can do the same with river rocks if you want, you'll just need more silicone and more cure time.
I used a dremel to cut the pieces...I also sawed the ''bottoms'' off to make the bottoms of the pipes flat and able to sink into the substrate. Mine never showed any signs of wanting to float..guess the added silicone/substrate was enough weight. I also didn't coat the underside, again cause of being impatient so its blatantly obvious and ugly. lol
I'm sure you've heard it before but if you use silicone use the kind not treated for mildew. It will usually clarify on the tube only if it IS treated, so be careful.
If you have any questions on hdpe pipe floats. We will give the professional answers to your questions.