A scratch built control unit differs only from a converted unit in that a non standard container is used. For my tw examples I used small wooden boxes acquired from a shop selling budget craft items amongst other products, Works.
All the electronic internals were bought throguh Amazon Prime or eBay there no longer being any high street shops selling such parts. At one time there were independent suppliers, then there was the US company Radio Shack that traded under the Tandy name in the UK and finally a UK company Maplins.
There were, and still are other companies that were mainly aiming at the commercial market and until the arrival of the internet produced complex catalogues. Interestingly through the internet these companies will trade in small quantities, but their prices reflect the higher specifications they require for products they sell.
Above are external and internal views of a single unit. This has a built in voltage and current meter reflecting the load being controlled. The DC supply unit is also shown.
Below is a double unit with resettable cut out switches. These are for use in boats and caravans where 12 volt systems are used. The box has been stained black purely as a finish.
One area where sourcing has proved difficult is the control knobs. Usually the units come supplied with a small knob which actually works fine although a larger knob would seem to refelct the control aspect more. Currently sourcing larger knobs is work in progress.