I often make my own twee lab hardware - logic probes, op-amps (I love op-amps), power regulation etc. Not production quality, but that's fine.
There is so many modular parts available that this is easy. LDO regulators usually have a control pin and a resistor divider to adjust their set point. Microcontrollers have builtin clocks, reasonably wide power tolerances and pretty durable inputs. They have a good set of bus controllers (rs232, i2c, spi, parallel, etc), and useful peripherals builtin (pwm, ethernet, ADC, etc). And they're pretty easy to program. Ditto for the peripherals, like DACs, memory, clocks, SD cards, and the like.
And op-amps? Op-amps are lego bricks. It usually takes a couple, but you can combine them like math equations to create a lot of different things. Differential probes, amplifiers, PID controllers, and so on. The trick ' and I'm not good with this part ' is that it is hard to get high-bandwidth, and stable, and noise rejecting, and non-distorting op-amp designs. Just like real math.
But that is ok, because most often I'm looking for a specific kind of measurement, not too accurate, and not too general, and not too expensive. Otherwise it's worth it to buy pricier lab equipment (like a bench power supply, oscilloscope, data loggers, isolated probes, and the like).