Rather than purchasing a mount that was already made for my CB antenna, I thought one I fabricated would be sturdier and more suited to my needs than a store bought one. The mount was constructed with things lying about my shop and a few purchased goods. That being said, use my Instructable as an idea and put whatever parts you have to fabricate. There's a million was to do one thing, so try your own way!

Step 1: Parts

Parts required for this project:

CB antenna
Antenna spring (if you wish)
Antenna to coax connector
Coupler (Antenna Thread)
Right angle bracket
Metal washer
Nylon washer (Usually comes with coax connector)
2x 1 or so inch long 5/16 inch bolts with nuts and washers to match
4x Rubber washers or O-rings

Step 2: Tools

Electric drill
Assorted bits
Sawzall w/ bi-metal blade
Angle grinder
Spanners
Cutting Lube
Gaffer's Tape
Level
Sharpie Marker

Step 3: Modifying the Angle Bracket

One side of the angle bracket needs to be cut down in order to sit the antenna close enough to the truck box. This can be done with a Sawzall and bi-metal blade like I did, or an angle grinder or dremel with an abrasive blade. After the cut, the edges are sharp, so it is adviseable to grind them down. It also makes the edge look a bit more professional.
Add some gaffers tape to the back to help protect your truck box. Make sure to wear saftey glasses and gloves because sparks go everywhere and are very hot!

Step 4: Reaming the Hole Out to Fit the Antenna Mount

Depending what size of angle bracket you purchased, you may or may not have to ream out the hole in order to fit the antenna stud through it. Choose a drill bit slightly larger than the stud and drill the metal slowly and use plenty of thread cutting oil to prevent damage to your bits.

Step 5: Assembly

Start by adding the coac connector with the nylon washer on the angle bracket. Attach it by threading on the coupler, and torque it down with a spanner real tight. You don't want it flying off. The antenna attaches to the coupler, but we'll leave it off to make attaching it to the truck box easier.

Step 6: Drilling and Installing the Mount to the Truck Box

Mock up the mount to your truck box and test different locations. I found the best area for me was on the passenger side. Make sure the mount is plumb relative to the sides of your truck box and drill out the holes. Attach it following this order: bolt, bracket, truck box wall, rubber washer, metal washer, nut. Torque it down good, attach the antenna into the coupler and the coax into the connector and you now have a bombproof mount! If you decide to use an antenna spring, insert it between the coupler and antenna. Good luck!