go with ply wood, its cheap and easy to work with... actually the angle iron i normally use is slotted iron, its cheap and even if its too short, you can always cut it and liquid nail it together.... if done properly you wont feel the difference....
what i usually do is order the wood and have it delivered to the spot and just nail the box together... nails are cheap.... build the frame of the box first then fit the supports in after....
for the top im not sure if you can use a thick piece of acrylic, so it slidable....
remember to have brackets on the edges of the boxes to prevent the sides from splitting apart.... this is really important to ensure the lifespan of your box
in summary:
liquid nails
normal nails
plywood
hammers
angle iron
lots of homies to build the box with
good if your on a budget....