That's a really good question, and the answer is that nobody knows. We know a great deal about the plan of Roman forts and fortlets, but none of them actually survive beyond a couple of metres or so above ground level, so any figures given for the heights of fortlets is just guesswork.
There are lists available online of Roman forts and fortlets in England, Scotland and Wales.
One of the best preserved fortlets in Scotland is the Lurg Moor Roman Fortlet at Greenock which was located at the western end of the Antonine Wall. This short video gives an idea of what the remains look like today.
This video gives a 3D visualisation of a Milecastle on Hadrian's Wall. The video forms part of an online course offered by Newcastle University on the FutureLearn platform. It might help you estimate a figure for your model.