Insurers who provide temp cover
usually want to make sure that the risk level associated with the driver is at a minimum. Therefore insurance companies tend to frown upon more powerful cars because insuring someone for a short period and then having to pay for a huge sum to settle a claim is not very profitable.
Which insurance companies have you tried so far? There are not many insurers for short term insurance but if you tell me which ones you've approached up until now, then I can tell you whether there are others which you can try.