I did this three years ago, excpet that it was stashing my rock and beach gear on the way to Calcutta. Just ask at the hotel you are staying at if they can rent you a locker for a few weeks. The really cheep places don't offer this service, but slightly more expencive places will let their guest leave a bag locked up for a few weeks.
I had sucess at both the My House Guest House and the Sawasdee House in the Banglamphu neighborhood.
Have fun on your trip!