Materials for the handle and scoop.
Handle - wood, like a shovel handle. Then it floats upright and can be replaced when needed. Tubular metal as second choice, Fill it with expanding foam once complete.
Scoop- SS or aluminum. 1/16" thick. Heavy is NOT bad, if you build and design for ruggedness. If you build shoddily, it wont matter what you use.
DO NOT attempt to make a scoop from PVC - you will be let down eventually.
Shape
Round bucket with a prow point - NOT squared off - or rectangular with the same.
Hole size or combination of holes sizes.
NOT holes, mesh. You will have to design a rigid framework to support the mesh, which should be on the INSIDE. The advantage is that sand filters through mesh MUCH better than holes.
Holes, on the other hand, make a more rigid scoop and dont require designing a framework, which can be complicated and tends to weaken the structure. Therefore using perforated metal simplifies construction. Regardless, whether you use holes or mesh, size so a dime cannot fall through.
Size of the handle and scoop.
1.5-2" diameter handle
Scoop... 6" approximate
Bigger is better, too big is ponderous. 6" is a good median.
Length.
Chest high
Angle of handle.
Straight since wood is preferred and thats how you get them at the hardware store.
Angled back TOWARDS you if using metal handle; 20-25 degrees.
Accessories
Magnet in bottom to catch hooks and other sharpies.
Lanyard attached to your body
Handhold mid way down the shaft
This may come as a shock to you, but you dont have to DIY everything. Since you have saved a ton on the Aqua Ace mods, maybe a proven scoop design is in order, eh? I mean, would you try to design your own scuba regulator, just cause you fancy a dive?
All the mistakes you can make on a design have been made by others already - the water is a tough environment and those who have gone before have broken all the bad designs already. What's left is PROVEN to work.
But knowing that stubborn streak of yours and since you are adamant about making everything yourself, have a look at the Sun Spot or ProScoop lines and duplicate. Why reinvent the wheel? Here's a few links:
http://www.sunspotedm.com/beachscoops/thescoop.html
http://www.proscoops.com/