i think just write your own by word,
state clearly who and who,
costing,
first payment - date
second payment - date
third payment - date
late payment how how how,
if did't clear payment by how many month, then how how how
black and white shud be like tht okie already
and important both sign and got one third parties
if want more secure, find lawyer, but less less ppl will do so la ..haha