With your fluctuating water table I would just put 4" thick pea gravel in 4 different areas to start. I would put it on each of the compass points, in water levels (at full pool) from 1' deep to 5' deep. The BG will tell you what side of the pond they prefer and you can expand the bedding areas from that information.
Initially I'd only make an area about 10' wide.
To prolong the life of the pea gravel in the pond, I recommend putting geotextile fabric under the gravel.
Put the gravel on a shallow sloped area, 4:1 or less.
If you have dense cover near the bedding areas for the fry to hide in, that will add to the spawning success.
| 
