One thing I have noticed...If your going to add Metal Halides (even in the future) make your canopy front loading. This will help keep the top from bowing. I know of one person (who shall remain nameless) whose top loading canopy top bowed the first day. Also, several coats of polyurethane (sp??) will help make the top last longer.
If you do go with the front loading approach remember to make the opening larger enough to get pieces of rock through, otherwise you will be taking the hood off everytime you add something. (I got lucky there, thought of it the first time I added rock after I built mine)
tw