I made one for Christmas, not that one obvs, but doing the sponge as a big slab rather than biscuits is a good idea. I made my own biscuits because I wanted to have a go and that method would have been much easier and, I suspect yield better results than faffing about with a piping bag.
When I retire, and have the time, I'll make this:
https://www.youtube.com/watch?v=4-z6imVeNF8