Recommending using a Wago [1] or Set-Screw wire nuts [2] for joining wires as these are harder to use improperly than a wire nut. Video on how to use a wire nut [3]; people mess these up all the time. For the mains voltage I recommend using at least solid THHN 14 AWG wire, it's the minimum required wire size for a 15 amp circuit (Most outlets are rated for 15 or 20 amps); color coded as well, white for neutral & black/red for your hot, green for ground. If you're unsure what one is hot, compare the voltage to the ground. A good clean design should not require electrical tape for it to be safe if you take the cover off.

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/...