depends what reinforcements you want, it can be deck reinforcements, fork reinforcements or even bar reinforcements.
if its for forks, jam a decently think pole down the inside of your forktube so it sits in nice and solid.
for deck reinforcements, you can get 2 pieces of square tubing and drill them to the outside of your deck, i think its like 10mm by 10mm maybe?
if you want to buy deck reinforcements, go to www.skaterhq.com.au
The sizes for box reinforcements are 19mm by 19mm (3/4 inch). The length that it's sold in at the hardware store should definitely be enough for one deck.
All the top part of the L(turned 90 degrees clockwise) is just an inconvenience and more weight. It does not help in anyway because you dont bend pro decks.