Utilizing a SATA connector that is surface mount soldered to a vertical PCB as your only form of support = either you're saudi royalty rich or just plain too careless to deserve a $3k + computer.
Masking tape and compact discs is a ridiculous over expenditure of workmanship. Aside, any sort of tape...not so good an idea.
Those OWC shelves look perfect, but they're $20 a pop; and you already have a sled.
Traditional sleds won't work, because you'll need a sata cable, and there's just not enough clearance without trashing connectors/joints/cable.
But this is all mute.
Looking at an SSD, let's consider the connector side, positioned so that it's at the left (for mac pro connection) the top. The backmost, leftmost mounting hole lines up with the backmost, leftmost mounting screw on the Mac Pro sled. You'll have to remove (and save or toss) the leftmost screws on the Mac Pro sled and obtain the correct size screws for your ssd. Nonetheless, you've got one corner covered, free.
The leftmost right most screw hole on the sled will have to remain a "wash". It won't be used.
For the right side, you're looking for something like this:
http://www.hardwaresource.com/hinges/SPECIALTY+HINGES/Chest+Hinges+-+Box+Hinges+-+Lid+Supports/Toy+Box+Lid+Supports/7-1%7C2%22+Lid+Support
You can remove the extraneous hinge stuffs.
or this
http://www.strapworks.com/Metal_Slides_p/ms-f.htm
The backmost/rightmost mount for the ssd is flush to the sled's mount; so you can use anything that is between 2 and 3 inches in length (5.1 to 7 cm).
For the right most/top most, we have an angle to contend with. The distance is 2 inches (5.1 cm) center to center.
If you own a drill press, or proper drill bits and a steady hand; you can buy flat brackets at the hardware store. They may have oil or lubricant on them, however and are often made of unpainted/treated zinc. Not sure how that will chemically interact inside of the computer.
Buttt, if you own a drill press; you are probably aware that you can always order metal plate from companies like "speedymetal" or "onlinemetals", although, at such a small size, I'd just buy a stainless steel wall plate and adjust from there.
I recently grabbed some pre-drilled brackets from the hinge section of a hardware store. They matched up, so I'll chime in if it works. I'd have preferred a flat hinge ala my example, but know one around carries them for some reason.