I've been trying to think how to construct a rather large screen dome
to place over the BD to address this problem. ("Large" = reduced
suction per sq.in. = small critters (and fish) can escape more
easily); but screen grid still large enough to permit mulm to pass
thru.

Or perhaps concentric domes of decreasing grid sizes(?)

Unnecessary complexity.

It's a difficult question isn't it? -- trying to solve two intertwined
but mutually exclusive problems at the same time!

Do you think it's worth experimenting?

Sure. Why not?

I think that you might start with:

1. Perforated PVC sheet.
2. An ability to draw a circle and cut it cleanly.
3. A dome (to place your hot pvc over, to form it)
4. A heat gun.
5. Elbow grease, safety precautions, and common sense.

I think you'll do fine, and the fabrication won't be as hard as you
think. Here's what you're looking for; it's probably expensive, and
if you hunt locally you'll be able to get it for less:

http://www.usplastic.com/catalog/pro...uct%5Fid=10081

Also, since you are thinking of novel screening solutions, you may
find this interesting:

http://www.aquaticeco.com/index.cfm/.../8559/cid/2079

Note that it is ever so slightly flexible (firm but somewhat soft). Not perfectly
rigid. It may get slighty deformed in shipping (mind did), but is easily straightened
with heat application (such as slipping it over 4" PVC and putting it in the sun).

C//