A modular system does not have to cost a lot of money.
Before you can start, you have to make some decisions:
The design: Eurorack or determine yourself.
A 19 "rack has 84 subunits (TE), that is about 427mm in the Eurorack standard a height of 3 height units (U) that are about 133mm.
The modules are on average 30mm wide which equals 6TE, for most DIY projects 6TE are perfectly adequate.
The depth of the case:
If you later want to make your own modules on stripboards you should dimension the case sufficiently deep.
Mounting the modules:
You can simply screw the modules to wooden slats which are fixed in the housing at the top and bottom. Or you can use Z Rails which are available in 2 versions.
Once with loose nuts in the frame or with inserted threaded strips.
The first variant is better because then no ugly gaps remain if you do not use standardized panels.
The power supply:
Modular systems are powered by bipolar PSU (Power Suply Unit).
That means there is a positive and a negative supply voltage.
So plus 12 volts and minus 12 volts. The reference potential is 0V so you have a margin of 24 volts.
Power distribution within the housing, most conveniently tensioned bare wires to which one clamps the modules, or something more luxurious with so-called busboards. The latter are also available in a cheap version, so-called flying busboards.
For DIY front panels is very good 3mm thick MDF board. Can be processed well and costs almost nothing.
For a first case one can e.g. use the case of an old speaker.
Here is a schematic for a simple power supply.
THIS SCHEDULE DOES NOT INCLUDE FUSE - THEREFORE IMPROVES POWER PLUGS AFTER USE