Since the bed is stationary, would you think it possible to use a different heat source like Warm up brand heated flooring? You can make it exactly the size you want. Take durock, thinset and screw to wood plywood. Run the warmup wire on top of that, don't forget to add and check heat sensor before going any further. Make a frame around this as a box 1" higher than the wire, mix and pack mud (1:3 ratio of portland cement to course sand) use very little water. You want the mud stronger than usual. Take some mud squeeze it, should stay together than bounce it off hand if it breaks in large chunks than it good. Take stick wider than box and run across to make perfectly flat. Let dry, than take kerabond and glue metal to the dimentions of wood frame to lock everything in place. Slip your machine over this square. I use metal bed with kapton tape and hairspray (aqua net). With the bed pre heated I spray it and then hit print. By the time the extruder heats up the hairspray is just the right tackiness. When the print is done you smack it with a scraper handle and it pops off. After the bed is half cooled down of course. This trick I haven't had ripped kapton, I've been using the same tape for over a year. Wipe the bed with a wet towel after a couple times cause you get build up. I hope this helps with your machine. This way should give you the most even distribution of controlled heat. You could even take and enclose the whole thing in plexiglass as well, drafts and people walking by can mess a print up as well.