I got the Auber single unit and probe. Already had the SSR & HS. Probe broke so I got a better one. Box is a small 'experimenter' one from Frys. total cost for me was ~ 40$ US. No fancy connectors.
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That's a great way to start - or finish. This will get the job done and for a decent price. If, as time passes, a fella wants more convenience or more flexibility, "fancy connectors" and such can be added later. I ran mine for years with the TC connected directly to the controller. One day, I decided to replace my TC, as the one I had on the pot went wonky on me. I scrounged eBay for a better TC (for the application) and got two. Then I got the connectors and made better TC probe holders for each of two pots I run. That added to the cost of the unit, but it was spread way out and didn't hurt.
Your idea will get someone in the game on a budget, yet leave a lot of latitude to expand on the system over time.
Many of the components can be had for very reasonable prices on eBay because there's no big crunch on nerdie, tech stuff, like there is on shooting stuff.
Anyone using a SSR - they will need to dissipate some heat. Attaching one to a metal box is probably enough for this application. Plastic, maybe not as much, especially without a heat sink, but probably won't melt the box as much as shorten the life of the SSR.
That box Keith made is the most extraordinary heat sink I've ever seen. The whole enclosure is a heat sink. Nice work.