I'd look over both the homemade cabinet incubator set ups and the mini-fridge set ups in the search engine here.
Both have applications for that style and size of incubator. I run a mini-fridge and a wine-cooler one, so probably smaller than your application.
There are guys and gals using Light Ropes, outdoor christmas lights (LOL), flexwatt heat tape, regular light bulbs, under-cabinet puck lights, the insides of curling wands, you name it, for heat sources. If it generates light or heat - can be used. Don't use any form of spotlight/flood they get too hot and create hot spots.
Most people making their own go with fans for circulating heat and air. You can use the cheap 12v computer fan or invest in the heavier duty (less likely to ever burn out 110 v fans).
Whether you need a return air plenum kind of depends on your actual interior dimensions - be sure to include them in any air/circulation ventilation posts.
Heat fans air - something for humidity at hatch or when necessary. Trays for hatching or incubation. Whether you want to add an auto turner - premade, rig something you can turn from the outside, or hand turn.
You'll want to look into "heat sinks", and what people are using - they help keep temps stable.
Again - search the forum for homemade and incubators and you should come up with ohhh thousands of posts.
