There are several types of boot racks ( at least 3 that I can think of) I have the same rack ( sans bike and bike carrier) as the one pictured with the bike, I think these might have been made by Thule. There were ski carriers for these racks as I have see them for sale in the dark distant past.
Maybe Thule could help you out, they probably wont still stock the ski carrier to suit but might have suitable rack mounting clamps to adapt one of their carriers to the racks bars using supporting struts fabricated to suit.
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"