[bactrus]

Agree with Robert. Most lowlanders survive pretty well in just pure sand. So by adding some cocopeat and/or cocochips and/or laterite (which is found abundantly in Malaysia), should help water retention quality of the media.

Charcoal, delphiguy, is an excellent idea. Especially if you crush it into small chips. Has water retention qualities too. However, softwood charcoal and charcoal which are not cured properly should be avoided as it will encourage fungus growth. One point to note: charcoal production using primitive technology (like in Malaysia) is enviromentally damaging i.e. big carbon foot print.

Highlanders are beyond me be as I don't own any.