Hi Gill,
Depends what you want to achieve.
You don't say, but I assume that the cracks are in a section of plastic, usually either GRP (fibreglass) or an ABS capped plastic (like car bumper material)
For a permanent repair you need to stop the crack spreading with a fine drill hole - as already pointed out - and then excavate the crack (only a very small amount) and then fill with pre-coloured GRP or ABS repair kit as appropriate. These take some skill to use properly.
If you want a short term fix, then try the choices you already proposed. The creeping crack cure has been successfully used by Eriba owners with 25+ year old caravans to make a temporary fix to the seal between the window rubber and a fixed window. It really does find it's way into cracks and lasts for at least a year whilst you work out a permanent repair, but has no gap filling abilities at all.
Robert