sump ideas anyone?

[68shelby]

I have a 100g corner round tank with a way to small inadequate sump underneath it. With no room for any lighting for a fuge or any other cool stuff. So its got to go.
I also have a 50 gal 36x16x18 tank with a sump (downstairs) I currently use it to primarly mix water. Any one have some quality ideas on converting it to a sump for my 100g, It would have to include a refugium.
Or does anyone have a long 50g 48" they want to let go of for reasonable price that I can convert into a sump.
Any advise is welcome and appreciated.