Here is a little trick I use when clamping the top and bottom decks on my plywood boards. This came in really handy while I was in the process of building up my clamp supply. It really helps with the way I build boards and is particularly helpful if your top or bottom decks are thick and need the wood to bend a little further right at the edge.
The idea behind this is that the spacer you are applying pressure to will spread the pressure from the clamp along the length of the joint. The spacer evens out the pressure between the clamps and this creates a longer point of contact rather than just two very small points at the clamp location.
-Get a length of timber say a minimum of twice as deep as it is wide that will run the length of your board while bent around the rail edge.
-Tack clamp the piece you will bend around your rail at half way making sure there is enough length at each end.
-Clamp at even intervals working from the middle out to either end at the same time.
This piece acts like a beam, the deeper the spacer is the more resistance it has to bending and the smoother your joints will be. You will need less clamping pressure as well.
Now you must figure out how far you can push the timber you are using as the spacer to get the best pressure.
By using this technique you can push the spacings between your clamps out a little further.
I hope this helps.