All these interfaces use a separate audio gain control pot to adjust the SSTV modulation level. I use a plastic enclosure box which measures 3 5/8 x 2 1/8 x 1 3/8 inches. These boxes are available from Radio Shack,  Hosfelt, Jameco, etc. The box pictured is a model JAL-2 purchased from Hosfelt. 

As a general rule you should make the cables between the interface box and radio a length of at least 3 foot. Cables from the interface box and the PC should be 5 or 6 foot in length. The RS-232 Com port cable for transmitter keying is 5 or 6 foot length. I find it easier to determine what you need in the way of plugs and buy preformed cable with molded plugs attached. Even if you cut off an unnecessary plug and discard it you are better off. Before you solder the cables in place you should mark the cable ends near the plugs as SB Line-In and SB Line-Out, TX Keying, etc. I slip a 1.5 inch length of clear .25 inch heat shrink tubing over the shielded cable and insert a small printed label inside the tubing before I heat shrink it. Mount the pot in one end of the box and extend the cables out the other end. 

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HF SSB Transceivers

FM VHF Transceivers