Summer is a bad season for solar imaging from inside my home... Previously, I used a long platform plate to let my scope mounted farther outside, I even used a binocular L-bracket to push it one or two inches more outside... those were poor solutions indeed.
I just tried to shuffle the stuff a little bit, this time, I placed the platform plate between the tripod and my mount, so it looked like this:-
To do this, I've to reverse the Gitzo to LXD55 adapter upside down, to put a 3/4" screw from the bottom of the adapter to fix on a threaded hole on the platform plate. The LXD55 mount head is then fixed via a slot on the platform plate. This setup is very secure and stable, since the other side of the platform plate is indeed resting on the window frame. However, I have no way to adjust the polar alignment since that tripod stub is missing here. I should free up the Gitzo to LXD55 adapter in order to get back this function. To replace that adapter, I need to use another aluminium bar with a long enough 3/4" screw to fix it on the tripod. That will not look as elegant but it will work.
The original 3/4" plate of the Gitzo has too short a screw, since that platform plate is quick thick by itself.