Built one from scraps, odds and sods and leftovers from jobs. The only expenditure was on drawer runners, which cost about £17, plus the usual glue, screws, doms, paint etc. Had to make some changes as I only had 10mm ply. I also had a cnc'ed top (made some time ago as a spare for the bench) which was 18mm, and a slightly different size to the one specified in the drawings.
Apart from the ply thickness, the main changes were to the wheels (mine are from an old concrete mixer that died, and are about 240mm dia), and the locking bar thing I added to stop the drawers opening (I couldn't find a sensible source for the catches suggested, and in any case I wanted something more positive).
Made the top frame from solid rather than built up from ply. I haven't used T-track as it's bloody expensive in the UK, and I haven't made the extension tables anyway. Can always be added in the future.
Solid rubber wheels. Still got concrete and mortar snots on from their previous use - must clean them up a bit sometime. This shot also shows the bottom of the locking bar, which slots into a bit of ply fixed to the underside of the cabinet.
Detail of the top bit of the drawer lock (underside) - a bit of ply with a slot in it, a spacer and a couple of aluminium tabs to hook onto the cabinet top:
Same again, but in place - it just slides onto the ply top
The batten to hold the drawers in place is just a length of beech, shown here removed. The bottom drawer is very useful
Drawer bases made from 10mm ply are too thin, so I had to glue a bit of softwood on the sides so I had something reasonably solid to screw the runners onto.
Making changes resulted in a few surprises - the original drawings are absolutely spot on of course, and changing a component size usually means that something else has to be changed to suit. I've used it on site a couple of times and it's very useful.
I think I'd like to put a couple of small locking castors on the front instead of the feet, and I'm not sure that the handle is up to the job, especially when I pull it up into the van (my version is probably a bit heavier than the standard because of the 18mm top). The skids are essential if you want to lift it into a van, and I feel happier removing the three Systainers first, mainly because I'm doubtful about the strength of the handle. It stands up ok in my VW T4, with a few inches headroom to spare.