these inserts pre made really aren't so bad price wise if I was to go this route would take me 0 time to put together. I guess the one thing I think about is if the stand is going to be a little over 4ft wide I am probably leaving some open space that I could otherwise use a longer tank/sump under to fill, not sure on the dimensions on your stand on this picture aboveWhere are you located?
I run a 40 breeder sump with Fiji cube inserts under my 120. It's a great size. Doesn't leave a whole lot of room under the stand, but enough.