I went on a "dry run" for the sportive, and ...

After getting a wet butt while training on Hoppy, I have refitted the front fender (just a 3 bolt job), and made a temporary "training" fender for the rear (the rear fender is a lot more complicated to refit - the "training fender I can just cut off on the morning of the event)/ Hoppy only seems to have one aero "horn" in this picture ...

... because one fell off in training. I guess the aluminium of the
tubes is prone to wear!

Tape and cardboard under the rear carrier
makes an adequate (and fairly effective)
training fender.

Disassemble detached "horn". reassemble, using gorilla glue to "fix it".
I used alignment marks I made on the mount and the tube of the "horn" to ensure I had them lined  up right before I got the glue out.

Hoppy came with an Altus rear mech (shown right), but I have had a M590 deore mech (shown left) in the cupboard (brand new in box) for a couple of years. The Deore mech is lighter than the Altus, and has a greater range (it takes 36t rear gears, and a 45t total difference), AND the screws to take the cage apart are better placed!

Setting up the mech.
At this point it won't even go onto the 32t cog.

No cable ends to hand, so another touch of gorilla glue
will keep it all tidy.

All present and correct. Chain freshly lubed. Hoppy now sporting a deore rear mech, and a 175mm front crankset.

Aero " horns" stripped and glued. the day after the pic was taken, I set another PR for a Strava
segment (fairly straight, modest gradient at the end) using these "horns".
They seem to be worth AT LEAST 5 kph (call it 3 mph), and it could be more -  a lot more.

Marginal Gains. A haircut saved me 33g
(I know the scales say 36, but i forgot to "tare" them,
and the bag weighs 3g!)

The mounting stub for one of my "Aero"
horns weighs in at 78g

the "tube" from a "horn" weighs in at 43g

A complete horn, with shorty grip, weighs in at
170g. I am using two, so that is 340g heavier (!)
the effect of the "horns" is so marked, however,
that it is 340g well spent.

Hoppy now sports a Deore M590 derailleur

The Altus supplied with the bike weighs in at 22g
more, and has a more limited range, and the Deore
shifts better. I picked the Deore mech up, BNIB,
two years ago for a tenner (about $15), and this
is the first time I have used it.

How did it go?
This is a Strava segment for the main part of the route - from the stadium gates, round the official 100km route, and back to the stadium.
Last year's time is my "actual" time from the sportive, and correlates with my "official" time on my finisher's certificate.
This year I was a LOT faster in practice than I was for last year's actual event.
A bit of weight saved (off me and the bike), and a bit better aero, including the "horns", too