Best way to secure a bike is rails of some kind so it doesn't slide left & right, then tie-ing down the bike on the non-suspended parts - i.e. rear swingarm and the fork crossbrace .
Front wheel clamp + rails is also quite popular.
These clamps need to have some space around the front wheel to work though - could be covered by your streamlining .
A popular way to transport awkward stuff around here, is to put an old mattress between the item and the side of the trailer and the item, then pull the item against the side, sandwiching the mattress - works only if the bodywork is sturdy enough for the bike to lean on.
How accessible is the seat with the streamline on ?
You might be able to route the strap inside the fairing if it's open on the bottom.
On ferries, they put the bike on the side-stand, put some protective cushioning over the seat, and strap the bike down with a strap over the covered seat.
Works surprisingly well despite the suspension still working some, and can be used in combination with something to keep the wheels from sliding sideways or fore and aft. (Straps or wood blocks nailed down in strategic places.)
I've also used this technique with the bike on the centerstand .
In this case you eliminate the suspension working on the strap, but you need to pull it down in front of the center stand in this case, and make sure the bike can't slide forward at all.