If you look through the very long but excellent ‘Let’s see your DSLR scanning setup’ thread you will find a great many ingenious and impressive ways to hold your camera relative to the slide or negative that you are copying. However one thing you come away with is there doesn’t seem to be any ‘go to’ copy stand so many have had a go at making their own, or possibly adapted an old enlarger (even a stereo microscope in one case). I think the problem is, as you have found, that ‘entry level’ copy stands that you can buy now are not up to supporting a heavyish camera and lens combination (sometimes not even a light camera & lens combination!). Often there is a ‘pro’ version but these can be pretty expensive. I use either an adapted Durst enlarger column or a Bowens Illumitran and these are hard to recommend as they are both a little hard to find and the Illumitran has its own issues as it is designed to be used with its built-in flash rather than LEDs.
One of the simplest ways to get into camera ‘scanning’ is the Nikon ES-1/ES-2 holder which is designed to fit on to the Micro-Nikkor 55mm or 60mm and of course a full frame Nikon digital camera, effortless alignment every time. With some ingenuity and a selection of 52mm extension tubes you can make it fit other macro lenses or cameras and so do away with the need for a copy stand altogether, but only for 35mm of course.