These are very handy - has everything but the relays you would need in one neat package. Controller, cord, box, transformer, fuse holder, master power switch. Has nice rate programming for jobs where you want to heat the item gently. Can easily handle up to 6 SSRs. Seems for heating elements that size you'd want to go with 100A SSRs, using one for each element wherever possible, for some safety against a relay failure that leaves it closed.
https://www.ebay.com/itm/233025861414 * I bought this one yesterday, normally a dealer would want about $450, but they come up all the time in this price range. One time I bought an entire small kiln just to steal the brain and bury the body...
But I think you can put one together yourself just as well. The connectors they use in kilns are usually nickle plated steel or stainless. The wire is optimally fiberglass braid covered, nickle plated copper, stranded. Fiberglass gives you an extra 50f. iirc of heat resistance compared to silicone coated.