Instead of an original Arduino Mega 2560 R3 you can use a compatible "Arduino"; in the first case you honor and support the creators of the Arduino project and the further
development, which everyone who can afford it should do, in the second case you save some money.
Instead of a mounting plate made of e.g. aluminium or stainless steel, which you probably have to have made if you are not equipped accordingly, you can also use two mounting
brackets from the DIY store. It doesn't look bad and is much cheaper. But then: file off corners! Risk of injury!
To savely store the alarm time (even when the Arduino crashes) you can probably use the built-in EEPROM memory of the Arduino or maybe even the RTC module instead of the FRAM
module (with additional effort in the sketch).
Instead of the (electromechanical) relay for switching off the speaker, you can probably use more elegant (electronic) components.
Header strips, connectors and terminal blocks can be dispensed with; the modules or cores can be soldered directly and thus you save some money.
Instead of the "overhead" housing, you could use table, desk, built-in housings etc. or build it yourself.
Instead of the mentioned (rotary) switches, buttons etc. one could use parts that give the whole a different design.
A (magnetic) lock could be fitted to secure the housing cover.
You could use varnished boxes, varnish them yourself or paint them.