Once they start cracking, there is really no fixing them.
JB Weld makes some decent plastic repair products.
The best stuff comes from weld-on.
Swing by Paragon Plastics retail outlet in Santa Ana and see if they can offer you a good product. Tell them Cal sent you by. Should be a short drive for you.
1908 McFadden Ave
Santa Ana, CA
(714) 835-8181