Well, there are two problems here. First, you use the Lascal Buggy Board Maxi with an ultra-compact - the smallest pushchairs on the market designated for traveling and occasional use only. Using a small pushchair with a large (Maxi) kiddy board is certainly not a good idea.
Next in line, overall a buggy board is not a good idea with any ultracompact. Like, honestly, please read this guide that describes why and how ultra-compact strollers should and should not be used. You might damage it over time - and even more so since The Pact Flex is on the lower end of the market, price-point-wise. Here's our other guide to read - anarticle about the advantages and disadvantages of a buggy board.
Ok, well, I warned you... moving on what will fit if you really have to be using a sibling skate with it - certainly try a smaller buggy board. Either the Lascal Buggy Board Mini, or maybe a universal Bumprider (comes in only one size). Do not put any seat on the buggy board and do count on worse maneuverability and center of gravity - the stroller, in any case, was not meant to be used with a kiddy ride.