Locks that work

depending on seatpost length maybe thread the shackle through the seatpost rails (towards the back) and lock the crossbar under the toptube/inside the main frame triangle.

It won't fit :( not enough room between the saddle and the rails unfortunately. The holster thing, was it one of those fabric horse ones made from seatbelts? I was looking at those but wasn't sure how it work with the longer shackle, but if you reckon it's alright I migh give it a whirl.