John, I like your radial plugs... Good creativity...
Strapping the antenna to the wood shed will not affect the tuning...
So what are you guys going to measure? Gain/impedence/current drop
across the inductor/?
Are you going to elevated radials?
There are a few easy ways to make capacitors... Early on they lined
mason jars with foil inside and out... Not tuneable but can be
swithced in and out to provide the bulk of the capacity and then a
small variable cap in parallel for tuning...
(I only thought of the banks of mason jars the instant I typed this...
I don't know what the capacity would be but they will take a heck of a
lot of voltage before arcing.. I will have to do that at home and
measure)
Anyway, you can stack up two or more 8X10 copper PCB and put Saran wrap
between them Won't take a lot of voltage but may be OK for 100
watts... You can order thin sheets of Teflon to insulate the copper
sheets... This will make a high voltage insulator for a capacity
pile... You can adjust the capacity by sliding the plates across each
other... Another way to tune would be a hinge so the plates are like
butterfly wings...
I vaguely remember someone, somewhere, published an article about ahigh
power tuner he built with the capacitor being two copper sheets
separated by thin glass... He used a threaded rod and a gear motor to
slide the plates back and forth... Had limit switches, automatic motor
reversing, yadda, yadda... TOo much work for a quick-n-dirty
experimentor like me...

Another way would be two aluminum tubes that just fit inside each other
with Teflon insulator... Sliding these in and out like a trombone will
change the tuning... I have actually done this.. Keeping the internal
spacing even is difficult and the insulator seems to want to bunch
up... Probably a pile of plates is better...