I built a redneck pylon on my boat, but if you're not in to the DIY and want something more pro looking I would go with a swivl-eze pylon.
What ever you decide, just make sure it's supported good.
You'll have to weld the base to the bottom of the hull on an aluminum boat. You may be able to anchor it to the stringers as well.
Click on image in my signature, around the end of the thread there is a explanation of how I did my pylon install.
I would look at one of the pre-fab ones on the market as they should be well engineered. If you're going to make it yourself then I would overbuild it. I would still go for a manufactured one or hire someone to do it for you from a safety standpoint.
A good prefabricated pylon will cost over $200 easily. Add in the cost of strengthening the floor, building your own for around $50 seems like a good savings.