TENS unit that doesn't auto-shut off after 15 minutes

My TENS unit is great overall, but it has one irritating flaw. It powers down automatically after 15 minutes, forcing me to restart it manually. Picture being restrained and wanting the treatment to continue—I’d rather not have that interruption as an option for stopping the discomfort. Does anyone know of a budget-friendly TENS device that doesn’t shut off on its own after those 15 minutes? Or at least one where I can set it to run for around 60 minutes? I’m aware there are TENS units made specifically for adult play, which probably work well enough, but two things bother me: a) they tend to be quite large/heavy, and b) the price is often on the high side when you consider that (it’s still just another pulse generator unit after all).