I've done the same and love the large surface and multi-duty, since I have a small space. Although I love your fabric choice, I've chosen to use cotton canvas instead. It really helps "grab" the fabric to minimize distortion during pressing of blocks. I did make a portable board also, and opted for bright pink canvas
Mine is MDF board and to answer the question of heat transfer, I have not had any problems and have been using this board for many years. One layer of 100% cotton batting is all that's needed, as a fluffy surface will distort blocks also. Good Job and Happy Ironing