Many thanks for the replies.

Collectively, you've answered all the questions and a few more I'd thought of since posting.

I can squeeze in a 8x6 which should be plenty big enough.
I'll definitely avoid horticultural glass and maybe go for polycarbonate as I've read somewhere else you can seal the panels with silicon sealant to stop them moving too much.
Yes to bubble wrap.
Yes to auto vent and louvre window.

Off to find one that fits the bill (at the right price too of course) :-)

Thanks again