I used to take an ear thermometer in the tub with me, dunk my head, check temp in both ears to make sure I hit 102, then wrap up in bedding, fully covered, for 20 minutes or so.

Edit in reply to your edit:

Yes I also preheated water on the stove and had assistance from a family member, though technically no kettle was involved.

There are no doubt better methods if you have scads of money. If you don't, a bathtub and big pot on the stove, plus blankets, can readily get you there for cheap.