Any high-quality unit should strictly have a front surface mirror. Otherwise you get a secondary reflection. I know that replacement front-surface mirrors are still available for the top end Peak models. But in such cases, the exposed silvering is fragile, so you need to clean it very carefully. But if replacing one, even an error in adhesive thickness, if present at all, could compromise acute focus.
Bill - what is "just plain wood"? Try that expression out at your nearest lumberyard. They'll probably show you a scrap pile out back, intended for fireplaces. What I found highly useful was an early Chinese attempt at very high ply-count finish plywood. It stayed very flat due to these many well bonded thin layers, but was otherwise a marketing flop because the whole point in cabinet work for this kind of product is for sake of easily shaped exposed edges without voids, and in this case the exposed glue lines were bright red. Oops! But that oversight is no drawback in a darkroom application, especially for something stained completely black afterwards. I repurposed a scrap of that recently for yet another darkroom application. I reserve my real Euro finish birch, or maple, or mahogany for actual cabinet or furniture applications, especially now that materials are sky-high due to pandemic distribution issues.