Hello Niels

> I think I will try my hand on DIYing a latching manual gate.

If you are able to build circuits on breadboard pcbs then my sync2clock
converter provides most parts required for that:

http://fa.utfs.org/diy/a100/sync2clock/index.html
http://fa.utfs.org/diy/a100/sync2clock/din2clock.png

You need the part around the Q1, IC1, Q6 completely and IC2B. Either you
replace it by an NAND or you use the unused gate D of IC1 as inverter
for the signal. You may change C1 to 10nF to have a shorter pulse (=
less delay).

Florian