Hand Warmer Heater circuit . Need tips for element
Some nichrome wire run at low current could be made into an arbitrarily shaped element.
Conductive thread of some sort may be the best option if you can make it work.
Article on heated socks using conductive wool here.
Link to circuit used for heating thermochromic ink on fabric.
At this temperature a few resistors could easily be used too.

The Nichrome link, and resistor datasheets will give thermal data. 
Here is an example from a resistor datasheet:



This can be used to work out the voltage you would need to apply to dissipate the power for your desired temperature rise.
For example the CFR25 size would need to dissipate 0.125W to reach 25 degrees C above ambient. This is assuming the ambient is 20 degrees for a total of 45 degrees.
Say your supply voltage is 12V. For 0.125W we need 0.125/12 = ~0.010A. So the resistance needed would be 12V/0.010A = 1200 ohms. 

For a ready made solution, a heater pad like this could be used.
Various other sizes here.