Click a square to block it. With symmetry kept, its opposite is blocked at the same time, which is what a published grid almost always does.
Attempts before it stops and says so. A grid that will not fill quickly usually will not fill at all, and knowing which is which is worth more than waiting.
The list is yours, not ours. A good word list is most of what makes a fill good rather than merely legal, and a good list is somebody's work. The demo list here was written for this page and runs out past seven letters — paste your own for a full-size grid. Nothing typed here leaves the page.