My son went to Borneo with World Challenge, they had a similar recommendation on sleeping bags
We bought a silk liner, rectangle shape for legroom, grey not white so not to show the grime – he slept in that.
He took a cheap sleeping bag from Aldi, it was £15 and lighter than decent synthetic ones we had. We knew it didn’t need to be v warm. It was there to show willing on the kit list it came back used just to sleep on.
We were nervous doing this, but a couple of his friends did similar and we figured at the time they were 18 years old and could find out how to buy warner sleeping bags over there if they needed them.