Angus made a tutorial about using pre-fab parts to build more complex models. In his example, he uses a straight bar to build a bridge rail. However, the same principles would apply to making a frame. When you see what he's done, it should spark ideas for what you want.
http://cownap.com/~mtmg/binedit/cube.html
Another thing you could try would be to find a frame that you like that's close to what you want, and then edit the parts of it that need adjusting to make it fit the project you have in mind. When you say something doesn't go down far enough, what do you mean? Is the whole frame too high, or do you want the lower end to be down even farther? If you say what it is you want to do, then maybe we can advise on how to go about doing it.