I have just invested in a dolly to attach to my tripod that I purchased from Amazon. Cost £35 and worth it. Good quality castors on rubber-wheels for a smooth ride all wheels lock and their are clamps to hold the feet of the tripod in place and comes with a carry bag to keep it in. I felt that I wanted some kind of dolly arrangement to add variety to my filming and add greater flexibility. The one thing I have noticed is the dolly can not compensate for rough surfaces therefore I need to build a track for my dolly to roll up and down on.
I have used 2 x 2m lengths of square guttering mounted to 3 x 100mm x 25mm x 700mm timbers for the track and ‘sleepers’ and I have modified guttering brackets to secure the guttering to the ‘sleepers’. The only problem is that the castors keep wanting to spin when pushed or pulled in the guttering track; so I need to secure two caster by linking them together using wire or slotted metal so the can not turn the third wheel running in the opposite gutter should be okay and if not I can find a way to hold that in one place as well.