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Rock substrate to place in experimental flume

I plan on making an insert that goes into an experimental flume.  The frame of the insert will be about 3" x 24" and will be constructed of acrylic.  Onto the surface of the insert i will affix a rough bed of stone/rock.  Stones will be ~ half-inch diameter.  Does anyone have guidance as to what type of material (epoxy?) I should use for the base of the flume into which the rocks will be placed?

Tricky if you need lasting results....
The water will tamper with most epoxies over time, especially if you add UV...
Acrylic can be "welded" using Acetone, so it would be quite easy to use a thin fibreglass sheet.
Cut and place the sheet on the acrylic and use a brush to soak it with some acetone.
Let fully evaporate and dry, then test if the bond is good in all areas, if not simply repeat until it is.
This will result in bonded fibreglass on the acrylic that is still dry and without "plastic" on the top surface.
Now you have a huge surface area available for attaching the rocks.
Best of course would be to sand/grind every stone to provide a flat surface for the attaching.... - but never mind unless you have lots of time to waste.
Fibreglass resin would be an option if you choose one that is fully waterproof (by submersion and without paint on it).
Casting resin is clear if you require a clear result.
Another alternative with the fibreglass sheet properly bonded is to simply use cement.
Fill the flume high enough with a fine cement and sand mix as directed on the packing to have the stones about 1/4 to 1/3 submerged.
The cement will bond nicely to the fibreglass if it is a too fine mesh - the water/cement slurry must be able to soak it - it in doubt use a wooden dowel to tap the entire surface after putting the mix it.
This removes all air bubbles that might be trapped in the fibreglass.
Using a stiff brush on the fibreglass prior to using the cement will help if the sheet is really smooth on the surface.