The Zeptomag Magnetic apogee detector is very small. You can adjust the angle of detection/event and the unit needs 1.5g detection before it is armed. It also has a backup channel that you can set to fire to whatever time you wish after the main charge fires. The unit is sold by Zeptobit, the manufacturer is based in Norway. The unit costs $59. I have many installed in my rockets, also have four of them I can move around between nosecones like the Intellicones that PML sells. I highly recommend them.
I have them installed in 38mm tubes. One thing, you can’t use all-thread unless it’s nylon or brass. Ferrous metals activate the sensor, so if you do use ferrous metal, it needs to be at least 4-5 inches away from the unit. I mount mine at the top of a fiberglass sled with any metal near the bottom of the electronics bay.