Dear Stan,

I don't mean to be picky but your question is based on a host of
assumptions -- the use of T-capped spar vs a C-channel, the aluminum
itself, and so on. Having created that particular box the odds are
overwhelming that existing solutions, such as two strut-attach straps
installed upon the shear-web with spacers & thru-bolts will prove to be
the most practical solution. Of course, then I have to define
'practical' :-) But given the venue in which you've broached the
question a fair definition would seem to be 'practical' in the sense of
a relatively inexperienced metalsmith fabricating a single copy of the
design in a home-shop type of environment.

Your chosen box also implies certain factors of cost and
serial-production that might be considerably different for a home-built
airplane.

For solutions outside your particular box you will probably have to
alter the box itself, such as considering a C-channel spar, or perhaps
one fabricated with stock L-type extrusions for the caps. The Zenith
CH-701 offers nice example of the latter in a design optimized for
one-off production in a home-shop environment. In that case, the
strut-attach fitting is little more than another, single, piece of
extrusion attached upon a doubler on the forward face (ie, the
unflanged side) of the shear web.

I'm sure that isn't the answer you were hoping to hear :-) Please
don't read it as a condemnation of your chosen design (ie, T-shaped
extrusions, etc).

-R.S.Hoover