I’ve been thinking about doing this for a while but kept putting it off until about 2 hours ago. I wanted a vintage biker scout to sit correctly with bended knees on his speeder bike. Very easy custom that I think turned out ok. First I cut out the plastic behind the knees. Then sat him in boiling water for 30 seconds before glueing his legs where I’d cut them. Unfortunately had to hold them for a while but you could always use some clamps or pegs etc.