a low, WIDE planter

Hello, this is a new area of endeavor for me, but I am looking for a mold to make some really large, very low planters. After going nearly " brain dead" searching the Internet the molds I keep finding are narrow and taller and I want low and wide. Surely there must be a mold out there or else very explicit instructions on how to make such a mold suitable for replications. I am thinking a gradual oval shape about 1 foot high and four foot wide.

Thanks for your attention in this matter,

Gene So