I would be very cautious about using watertank instead of proper dome.
The problem will be when the opening is cut - the tank supporting structure (actually, there is none) may collapse and you may end with wobbly and useless pile of plastic.
Instead, we (I am also considering all sorts of options for my future observatory) may try to talk Col into manufacturing some more of his very interesting 2.4m or 3m metal dome
http://www.astrodomes.com/ (he is looking into retirement right now and the availability of his products is pending on sale of the business).
Geo-dome approach is also very interesting one.. I am looking into sourcing the criitical elements of the construction, mostly the connectors, like in attachment (5-way connector is shown, 6-way is also needed for 3V dome.. something like on other attachment.)
Some interesting links:
http://www.strombergschickens.com/pr...plate-Dome-Kit (2v dome)
http://www.geo-dome.co.uk/article.as...=gd15_dome_kit
http://www.geo-dome.co.uk/article.as...e=GD5_babydome