Here are some simple model examples. Please feel free to use them, but only subject to the disclaimer that you do so at your own risk, verdigris makes no representations as to their correctness, and takes no responsibility for their use.
Simple debt corkscrew.mortgage style
Simple debt corkscrew.equal principal repayments
Sickness decrement model: We put this together for interest to understand what sort of drivers the rate of infections for COVID19 has. The model is extremely sensitive to changes in assumptions (of which there are several). Due to the sensitivity, and to lack of available data for determining the assumptions, it probably doesn’t make a good predictor for infection rates. The influence of a future vaccine is also ignored. If you’re interested, play with the number of contacts per person per day, and the probability of transfer given contact with a sick person. It’s clear from playing with these just how sensitive the infection rate is to very small changes.