You have several issues from what I can see. Use button head screws,
a good fastener house will carry them. Your main point of weakness is
at the screw location where the torsion will cause stress fractures
and weaken the the aluminum. Try to find scrap anodized aluminum and
cut out a piece from that. Aluminum will corrode when not anodized. It
makes a salt on its surface and will eventually reject the glue and
corrode around the fastener from the uneven charges.
I would consider a piece of titanium for the plate. Just search on
Ebay for titanium sheet, it is MUCH stronger than aluminum and highly
corrosion resistant especially for the salts and organics that come
from the roadway. It is strong as steel with less than 60% of its
density. Search for titanium sheet on Ebay, enter 310062160956 into
the search field, it looks like a likely candidate. It is .107" thick
which is very strong, which you need as the bending forces on this
larger wheel is much higher than the smaller wheels.
Think triangle on the bar design. The racking on the frame will
crack it over time if twists too much so take the support further back
in the frame. Airplanes suffer from this and they only bend a little
compared to the twist forces in your current set up. I would secure
it in three or four points. Screws top and bottom in the gaps between
where the wheels meet and two out on the tip of the frame. This will
stiffen the frame considerably.
You need a good safety factor on a design like this. If you are
going down the road and hit something, it could cause a bend on a
stress fracture, which in turn allows the wheel to contact the
frame....oops. So a light design that works today, may be a disaster
tomorrow.