I believe the gravel in the bottom of the beds of ewest's beautiful pictures is actually due to "kinetic sieving".

This is a well-known phenomenon when studying sediment transport that is the result of moving water.

I suspect the material just off the shoreline was initially composed of poorly-sorted sediment consisting of a size distribution ranging from pea gravel to fine sand to silt. When the BG fan the sediment to create spawning beds, they are only capable of creating enough water movement to put the fine sand and silt into suspension. The gravel gets left behind in the center of the nest and concentrated as the depression gets deeper.

If you look closely at the second picture, you can see that the spaces between the nests are uniformly gray with zero observed coarse material. That is the "fines" that were fanned out of the nests by the BG.

If you look at the water's edge in the bottom of that picture, you can see some of the "unsorted" original sediment that exhibits some widely-distributed bits of pea gravel.

I am not a fish expert - my background is geology. But I believe our conclusion might be, that the material that is placed for BG spawning beds does not need to be perfect. If you get it close to their needs, then the BG are capable of doing the rest.

One of the cheapest aggregates to purchase is typically called some version of "river sand". Depending on the source, it should contain a fair percentage of pea gravel combined with finer-grained material.

I would think you could get a dump truck load of that for a few hundred dollars (+ trucking). Spreading that with your FEL or skidsteer over the fabric should make for appropriate accommodations for amorous BG.