In reply to Rhys and Toby:
Thanks Rhys, don't know why I didn't see that before. They look like they'd fit. But I looked into it further and found other tapered beds of the same construction, including the Pacific Outdoors one that Toby linked to. However, it's interesting to compare the R ratings that the pads get for their insulation value against the price. It would seem that Thermarest have the lowest R at 3.2 for the highest price at $129 > $179, whereas, for example, Big Agnes pads have 4.1 for $85 > $99 and Pacific Outdoor have up to 4.4 but I can't find a price at the moment. Exped seem to come somewhere in between all that. Need to do more investigation...