i just bought and built a mini-ramp (http://www.skaterscafe.com/showthread.php?t=88382) following the ramptech 4' half-pipe plans. i would highly recomend these plans. they are easy to follow and well engineered. the way they have designed and built this ramp is really quite soild. it uses 2x6 instead of 2x4 and more screws than you can shake a stick at.

the plans are for an 8' wide ramp, but include instructions on how to build it to 16' wide. i built mine to 24' wide, it took a slight bit of adjustment to the plans (ie, some smaller pieces can be made larger, etc.) to build it for 24'.

there is one thing that i would say that the plans are missing and should have included, and thats to tell you to put concrete or something else soild inside the blocks that you build the ramp on for extra stability and that its a good idea to use brackets to attach the ramp to the blocks for even more stability (especially if your building a smaller width ramp).