Nice I upgraded to a 75 g a few months ago and also built the stand. I went with a 2 x 4 frame so I would have as much room as possible underneath for the sump/ components and all the other stuff... As for the top of the stand I used plywood and then put a piece of foam board for the tank to sit on. They sell it in 4x8 sheets in the building materials section at hd. I went with 1/2 ".
With the weight of the stand and tank I highly doubt you will have it tip over on carpet. I think mine full now with rocks and everything is over 1000 lbs.