Here are some little tricks you can use when you are trying to clamp something bigger or longer than your available clamps.
‘Joining’ clamps: Sometimes it is possible to simply clamp one clamp to another, with their business (working) dogs to the outside of your work as with pipe clamps, but in the case of door clamps this is hard or impossible to do since you can not rotate the dogs of the clamps.
Once neat solution is to use a piece of scrap as a ‘joiner’, sandwiching it between the clamp dogs. You need at least three clamps to do this, and more are better -
See photos below:
Another way is by using wedges, which seems simple enough, but it is amusing to me so many people do not think of doing this, which can be adapted to almost any length or shape with a little thought. See illustration below:
A prime consideration using either of these methods is that you should also clamp your stock to a flat surface or planks to prevent it from bowing up (or down) in the middle as you apply pressure to the outer edges of the stock.
Another consideration is to make sure your clamp dogs are well padded to avoid damage to your stock. With wedges, it is imperative you use a piece of sacrificial material between your wedges and your stock. Wedges are generally used in opposing pairs and it helps to blunt the narrower ends so you can drive them back when needed. You can easily make a simple jig to cut identical wedges on the table saw if need be. About a 1 to 8 or longer taper is appropriate.
-- "Never let your dogma get run over by your karma!"