I was wondering if I could use metal L brackets in the inside or outside the sub box the reduce the vibration or flex? I saw the wood ones, but those take space inside the box and my box is quite small and I don't wanted any more smaller.

How big is the box dimension wise?

2.8cuft ported. Quite small

Single 15" sub

I wanted dimensions, as you might not need bracing. I don't think metal L bracket would help any. You could brace the outside of the box with pieces of wood if its necessary.

All-thread is about some of the best bracing, easy to find, easy to use, fairly cheap, and takes up min. Space.

i've used some 1/8th inch thick 1/2 x 1/2 inch "L " brackets (really just some waste metal trim from the retail fixtures i built at a old job) i would say the enclosure didnt vibrate but all-thread, nuts, washers and loctite would probally do a better job