re#15 with a beam rack, you could fit the thing out to carry a
duffle bag,.. a side opening drybag version of the same.. on top.
there are racktop bags made for them too, add supports and there are
'spaniel ear' versions of those...

or as a support base for a large transverse saddle bag,

but there are lighter resolutions for those than a beam rack,
such as Carridice bag and their bagman support

the use of NEOS overshoes and platform pedals might be good.
then you wear the at the job shoes in them..

And i have a XL Burly best WPB cycle commuter rain jacket FS,
its way too big for me

and when the skys open up i get out my rain Cape