I use the "Be Focused Pro" app on Mac/iOS.
I use 30 minute slots and often "double up" so I just start the timer again when it goes off.
I don't use a timer for the gap, so often my breaks are longer. I don't care so long as I hit my goal for the day, which is usually 10x or 12x 30 mins, six days a week.
I've thought about the "deep work" issue, but the gaps are actually good thinking time as I wash the dishes. They enable me to make better decisions about how to use the next slot. Direction more important than speed.
The other problem with the idea of needing hours of non-stop work is that it's just really bad for your health. You have to get up and move around regularly anyway, stretch etc. so sitting for more than an hour just ain't gonna happen.