Thanks for the tips. This was my first time working with power components and I wasn't familiar with most of these parts. I'll incorporate these suggestions into the blog post so that they can be a resource to others.
Having a physical barrier between your low and high voltage areas is a must; the low voltage wire used to switch the relay is the only thing that should go inside the high voltage area (drill a small hole just for this wire & keep it away from 120v as much as possible), also mount the plug for your 5 volt source on this internal wall; or better yet use a USB outlet [4] as these are designed to be ran inside an enclosure. Using color coded wire for the low voltage is advised as well.
Since you're dealing with water, making this device GFCI protected would be smart. Have your mains run through this first before anything else [5]. And finally for the relay get one that is UL listed & opto-isolated in order to prevent bad things from potentially happening; the RIBTE01B [6] will most likely fit the bill. Be sure to check the ratings for your relays; putting more power through them than they are designed for might end up with your project in flames. If you're unsure about the power requirements, using a small relay to turn on a big relay [7] is OK.
[1] http://www.amazon.com/Wago-222-413-LEVER-NUTS-Conductor-Conn...
[2] http://www.amazon.com/Wire-Connector-Set-Screw-Black/dp/B000...
[3] https://www.youtube.com/watch?v=32z6Q8--fEM
[4] http://www.amazon.com/Leviton-T5630-W-Tamper-Resistant-Recep...
[5] http://www.homedepot.com/p/Leviton-SmartLockPro-20-Amp-Combo...
[6] http://www.functionaldevices.com/building-automation/display...
[7] http://www.amazon.com/Packard-C240B-Contactor-Pole-Voltage/d...
Edit: If you do end up keeping your solid state relay, ground the heat sink.
2nd Edit: If you want to avoid the need to 3D print the enclosure, using a 4 gang box [8] you could make this work. 1st slot is your input Single-Gang Decor Recessed Power Inlet [9]. 2nd slot is the GFCI [5]. 3rd slot is the USB outlet [4] with a cover [10] for your low voltage side. 4th is the outlet [11] to your heaters. Drill a hole in the box for the relay [6] if it doesn't fit inside as it is threaded and is designed to be externally mounted like this if needed. Mounting some lights [12] is helpful to know if the device has power and the outlets are powered. If you can find a larger gang device [13], you can incorporate an simple switch to turn the whole thing on or off and maybe mount everything inside.
[8] http://www.homedepot.com/p/Carlon-4-Gang-68-cu-in-Electrical...
[9] http://www.amazon.com/Midlite-MDT4642W-Single-Gang-Decor-Rec...
[10] http://www.amazon.com/Safety-Innovations-10408-Twin-Outlet/d...
[11] http://www.homedepot.com/p/Leviton-15-Amp-Duplex-Outlet-Whit...
[12] http://www.homedepot.com/p/Madison-Electric-Products-Smart-B...
[13] http://www.amazon.com/Indicator-Pilot-Signal-Light-Cable/dp/...