This is a tricky one. As a lawyer I would recommend you ask an attorney to draft one for you but I know that is not always budget friendly. I would recommend finding a similar type service and make sure you account for all the terms. Taking your draft to a lawyer for a review should not take more than an hour or two and will save you money in the long run. Strategically for an early stage startup this is probably the budget friendly way to go.

Let me know if you would like to discuss more,
Thanks,
Madhu