Hi Dec,
Seeing where you're located, the Hondaline accessories topcases may be an option to consider. IINM, David Silver Spares is a source. Dunno, maybe you can even get them through a dealer.
PN info from my archives (IIRC there are other PNs for topcases with lids colormatched to some of the ST1100's colo(u)rs):
08L55-MV1-860 / TOP BOX BLACK/GR [20 liter]
08L55-MAJ-810 / ? [Topcase (tailbox), 35 liter, black NH-1]
08L42-MT3-B00 / BRACKET TOP BOX [Rack, Top Box Carrier and Stay Kit]
08P60-MAJ-800 / ? [Backrest, Topcase Pad for 35 liter topcase only]
I sure like my Honda topcase. 14 years and 120K miles.
Regards, John