Save your money. Unless you have a decent machine shop etc. converting that structure into a serviceable dome will end up costing more than a commercial fibreglass dome.
Next what would you use to support it. A cheap garden shed would not be strong enough.
I looked at building a domed observatory 15 years ago and decided that a purpose built observatory was the only way to go.
http://barry.sarcasmogerdes.com/obse...s/obsvatry.doc
Barry