I'm sure more folks will jump in but I'll start :-)

Other things to think about - how are you set for wildlife
around there? Raccoons specifically.

Vertical sides (no plant shelves) and a thought out ahead
plan for netting is a good idea (for leaves in the fall - do
needles shed all year round or just at one time?).

For predators you can also use electric fencing or motion
activated sprinklers. Results will vary.

I think the size sounds good and I'm all for deeper is
better. If you keep a hole open in the days you have ice
(if you do) you will be fine for winter care.

For plants you can put them on black overturned plastic *milk*
cartons (usually sold at office supply stores). I have my plants
on an island built of aged builder's brick - makes nice fishy
condos.

I'm sure I'm forgetting something.

kathy :-)