This is how I built my time lapse dolly, it was made with a whole bunch of trial and error, hopefully this will help you build yours.
A lot of the stuff I used I had on hand, don't know where I got it, or have no idea what it worth but I'll do my best at giving you a complete list of what I used and where you might get it.
This is very much a "prototype" build and posses very little polish, I would approach this Instructable as a very rough guide of what you should do, do not be afraid to substitute the parts I used for parts that you have or can get easily.
This is my first Instructable so I apologize for any shortcomings.
Step 1: Doin' Rails
I will start with the rails because I found it super simple to set the width of the dolly cart if you already have the rail made.
Parts and Tools (all costs are an estimate, I didn't pay close attention to costs because it scares me)
2 x 8ft 1" square aluminum tubing $30 each here in Canada probably much cheaper down south.
1 x 4ft 1/4 20 threaded rod $2
6 x 1/4 20 sex bolts...yeah (aka barrel nuts or Chicago bolts) $1 each.
6 x 1" steel hinges $2-3 a pair.
Drill bits and a drill and a 1/4 20 tap/ thread cutter, preferably a drill press to keep things straight.
Some thing to cut the aluminum tubing, I used my compound miter saw.
Step 1 Cut the rails to length
I originally used the full 8ft but found it a pain to move around and handle in general so I cut mine down to six feet leaving me with two 2ft pieces to use for other parts.
Step 2 Cut the cross members.
The overall width of my rails is 6.5 inches so my cross members are 4.5 inches long and I used 3 of them.. You make these any size you want, remember the width of your rails determines the size of your cart. This is also a good time to drill the holes where you will attach the tripods. Measure out the center of the cross pieces and drill a hole, I used a no. 2 drill bit then ran the 1/4 20 thread cutter bit into the hole. This will be the bottom of your cross members. test this on a piece of scrap first.
Step 3 Attach the cross members to the rails with the hinges.
I clamped it all together and then put the hinges in place to mark where to drill the holes. This is very important to get right so take your time and make sure everything is square and the hinges are flush and your holes are marked out correctly. After you have marked the holes for the hinges go ahead and drill your holes. You are going to want to drill the right size hole for the screws that came with the hinges. test it out on a scrap piece of aluminum, if the hole is too big the screws won't bite enough to hold it all together and the hole is too small you will strip the screw head or break the screw trying to get it in.
Step 4 drill the holes for the "clamp bolts"
These bolts are what make the rails rigid. You have to drill a hole big enough for the shaft of the barrel nuts to fit in. Mark the center of your cross members and transfer that mark to the side of the rails.
*IMPORTANT* DO NOT DRILL YOUR HOLE FOR THE BARREL NUT IN THE MIDDLE OF THE RAIL!!!!! make the hole as low as you can with out the lip of the barrel nut going below the bottom of the rail, you need the top of the rail to be clear for the bearing to roll on. See image.
Now you cut a length of the 1/4 20 threaded rod that will reach in between the two barrel nuts. You may have to trim it down or cut a new piece if it's too short, no worries this stuff is cheap. Make sure you get a good fit and you can tighten good, you don't have to tighten too much but you don't want any movement in the rail. You want to do this for all three cross members.
There you go! You now have a foldable rail!