Had a large panel built for my next painting. 72" x 40.5" Used 1/4" untempered hardboard and braced with 3/8" beech plywood. Cost was $200. Very reasonable pricing and a quality bracing job.
Have found that after gessoing large panels the board will look very warped, as much as 1.5" warp in the middle. Takes about a week to realign and dry/cure fully. As long as your panel is properly braced it shouldn't be a problem.