Any ideas for diy

Trelis "raft" ? Would like to see both...the store bought and DIY kind.
But till you come up/w different...driftwood, often is because it does. I find the lighter color ones are hardest to sink. Pehaps a y shapped piece would not turn over on you so might make a suitable foundation for a couple of plants(small).

Yeah, basically it attaches via Velcro to the planter. I got myne from hydrophyte a while bad and sold the rafts because I never thought I was going to use for riparium plants.

I was thinking about using styrofoam to accomplish this but am unsure because I don't want a white eye sore on my tank.

Spend a lot of time in Home Depot looking at everything made of foam plastic, dark colored. I'll bet you will find something you can adapt. Those are basically just foam plastic with lots of cut outs in an attractive shape. Not the easiest thing to do by hand, but possible.

Hydroponics stores sell foam floats very cheap I think.