Parts list - rough
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Back of cupboard - old broken tripod that cost $20 in the 70s - rip the guts out of it and saw off the legs, leaving enough of the tubing to connect to some solid hardwood legs. Fit a solid hardwood platform and vertical bolt to mount transverse arm (the only piece of pine in the project).
Chair graveyard (at my work) - broken gas lift chairs as long as gas lift working, and a few extra knobs with embedded nuts.
Out the back of the lawnmower shop near the dumpmaster - few more threaded knobs from the handle fold position of old lawnmowers.
Recycling bin - a few milk bottles from which to cut out the "friction bearings".
Hardware store - various bits of wood, screws, bolts etc, couple of strong metal brackets.
Local fitness world store - some weights (go back next day and swap the 2.5kg for a second 5kg when you observe your first calculation was a bit of an underestimate!)
Construction
Design as you go! Use lots of big strong screws and bolts. Cross fingers that the guts of a lightweight aluminium tripod will hold up some 20 Kg! (It does - I'm careful to fully extend the tripod legs to leave no slack in the mount.)
Ignore some basic problems like when you mount the two brackets together, the already drilled holes no longer line up horizontally - if necessary, tilt the binoculars on the central mounting bar to compensate! Mind you, even picking the best bit of pine I could for the transverse, it still had a small twist in it.
"Test Drive" next post.