I am sure I read a thread the other day where someone recommended a seat pad/cushion that you can pre-heat and then take out with you for the day and it retains the heat for a few hours. I seem to recall the brand was something like Stokke or Stovie but no amount of googling is being up what they linked to. They were quite pricey - about £80/90 each - but I know the recipient will get a lot of use from it as well as being thrilled with it when they open it as they don't know that such a thing exists