new qrd calculator available, called qrdude

[JohnyTheOne]

15 cm.

[collo]

In that case, I would go with three N17+4,9 panels

Leaving 3mm for the backboard gives a deepest well of 147mm.
Using 3mm wide fins and 28mm wide wells gives you the required panel width.
Shifting the wells 9 units to the left gives a symmetrical panel with a zero-depth well at each end, which should look good.

Diffusion from 896hz to 6142hz.

See attached zip file containing the qrd frile for this design.