Well.. I havent got the images from when i welded the frame, but the frame consists of 3m 40x40x4mm, SS235 mild steel.
Most other stuff on this project is scrap.. bits and pieces of stuff i have lying around..
Made the first idler:

i had a piece of 240x110mm 6068-T6 aluminium - this will make up most of the wheels, for smaller contact wheels i have some 7075-T?
Then i made the tracking mechanism


The idler is fitted with 22mm skateboard bearings, and a 12mm stainless rod was turned down to 8mm and threaded with m5/0.8mm and a small "cap" was turned so the idles stays on the axle.
The mechanism is made of.. well bits and pieces of steel a nut welded to the frame and a 1/2" bolt. ;)
then i needed an axle for the drivewheel.

i had this 500mm stainless rod in 25mm OD, some unknown stainless but turned really good.
the flange bearings i got from some other old machine i took apart hade a ID of 20mm so the axles was turned, 18mm, 20mm, left 25mm, 18mm.. the 18mm parts is for the drive wheel and later
on the pulleys.

and here is the whole thing somewhat assembled/mocked up so i could determine if the wheel would align.. they wouldnt so spacers in aluminium rod was made and the stainless axle was threaded M8 in the ends to the spacers would double as a compression ring, they are turned sligtly with an angle so when i torque down on the M8 bolts.. the wheel friction stick to the axle.. =)
This is how far ive got.. This has taken me.. uhm 3 weekends so far..9 days...
And yeah the spring.. is a boat spring for anchoring boats.. its supposed to take care of a 600kg boat, but well i can compress it by hand so. will suffice. =)