First off, a little CYA:

You use this device at your own risk. My experience has shown it to be quite sturdy enough for hangs and campusing around (weighing ~170) but I can't foresee all build/climber/hangboard variations so use your own judgment just as you would on the wall. The pull-up bar used as a base is rated to 300 lbs and due to the added leverage, dynamic moves might not be a good idea for climbers nearer to that weight.

Now then! This is a simple and cheap plan for mounting a climbing hangboard onto a commercially available ($30) Iron Gym doorway-mounted pullup bar a la the Blank Slate system. The cost for the materials should run around $15-20 and the hangboard can serve as a base for a more extensive training station that still breaks down and packs into a gym bag. Modify as needed to fit your hangboard.

Tools Required:

Power drill
Saw
Scissors
Measuring tape
Ruler
Level
Pencil
File
Gaffer's tape (optional)

For accessories:

Drill press and/or vice
Pliers
Lighter

Materials List:

3/4"+ pine board cut to hangboard size
5" length of thick 1" steel pipe x2
1" iron pipe flange x2
Thick 3/4" long screws for flanges x8
Thinner 1" long screws that will fit your hangboard's holes
Card paper

For hanging objects:

9"x 2" dowel x2
3" wooden ball x2 (I found some on eBay in craft supply)
4"x 5/16" screw eyes
10" length of 1/4" steel chain x4
3/8" stainless steel shackles x4

For suspension trainer:

8' x1" 200lb working load lashing straps x2
6" x1" schedule 40 PVC pipe x2
48" of 1" tubular climbing webbing

Step 1: Mount Flanges

The posts you'll be mounting the board on are approximately 16 1/4" apart on center. You can use the screw holes on the flanges to reference their center.

Start by drawing registration lines through the center length and width of the board. I set the Iron Gym down and traced the tubes so I could be sure of the distance between the flanges, then centered the flanges at that width and traced the screw holes.

After drilling pilot holes, screw the flanges to the board. Sink the screws halfway first to keep the flange from spinning out of alignment. Remove the foam grips and plastic caps from the Iron Gym posts, thread on the pipes and check to see that the board slides on.

Step 2: Mount Hangboard

This can be a little tricky to position but is mostly straightforward: screw 'er down. I used a combination of registration marks, a level, measuring from the board's edge and tracing the edge of the board til i was satisfied I had very nearly centered the hangboard. Take a little time and put some weight on it or use a clamp when you drive the screws. Don't worry, you can adjust the angle a bit later. If any screw tips protrude from the wood, file them off.

Step 3: Leveling

Screw the pipe sections on tight, slide the completed board module onto the Iron Gym's posts and measure the angle of the board (you can also put the level on the hangboard itself if it's flat). Cut 3" x 1/2" strips of card paper and shim the doorjamb to custom level your rig, then tape the shim stack together. The thicker foam on the ends of the bar can also be reinforced with gaffer's tape to prevent it from tearing and reduce door frame marks

Now your board is up! Get hanging or continue on to build a few more accessories. You can also make more board modules with different hangboards, crimp rungs or even t-nuts and bolt-on holds (may want to use plywood for that, I don't have enough experience to say so do your research).

Step 4: Hanging Objects

These cost a little more (~$30) but add a fun element and can be installed elsewhere for use more like traditional rings. By sinking heavy-duty screw eyes into some shapes of wood, you can create some more grip variety as well as another height level that is useful for campusing workouts. I had the hardware store install the eyes for a small fee, but if you have a drill press or vice you should be able to drill the pilot holes and sink them yourself. These are hung by simply passing the length of chain (~10", equal numbers of links, a bit shorter for those over 6') over the bar and using the shackle to connect the ends to the screw eye.

You'll have to bend your knees to use these in a normal-height doorway but in my mind it's just dab practice for bouldering. As for use, ab work like Frenchies with knee raises and L-sit pull-ups are some of my favorite exercises on the dowels. I prefer the balls in a wide position for pull-ups, you can find a nice level of slopiness with 4 fingers or pinch 'em from underneath (har!) for a wicked thumb exercise.

Step 5: Suspension Trainer

Suspension trainers are all the rage, but a TRX is $200. What!? Make one for ~$10 and get all that unstable goodness on a dirtbag budget. All you need are 2x 200-lb lashing straps, some PVC pipe and 1" webbing. I used Keeper brand, these are working load rated and able to support my weight for L-sits fine, go for a thicker strap if you think you need it.

Cut the straps 42" past the buckle and seal the webbing with a lighter. Create handles by passing the webbing through the pipe, around and back through so you can yank the webbing out for stirrups. Then tie a water knot below the buckle so that the end finishes toward the buckle. You will have just barely enough to go around the pipe and tie the knot, so expect a little finagling then snug very tight with 1"+ of tail. Never subject a water knot with tails this short to climbing loads. Visually inspect the knot to make sure the tail does not slip as the knot locks down with use. Connect the two long straps together with a 4' piece (adjust to your doorway) of 1" climbing webbing using tight water knots, then girth hitch the center to the pullup bar. You can use gaff or climbing tape to cover the bolts in the middle and provide a grippy surface.

Now you can thread the webbing ends through (out through the 'top') and use the buckles on the handles to adjust their height. This is great for pushups, rows, mountain climbers, planks and many more exercises to round out your on-the-go climbing gym lite. You can even put your feet in the stirrups (with some effort) for moving hangs to work on endurance.

Now get to training!