You can lay them down at an angle. TentLabs also sells regs (not sure they would fit your needs). IMO I would go with an external regulated supply. The amp looks rather small, so keeping heat out of it would be a consideration. A quick and dirty CRC followed by a 78xx and 79xx should do the trick. Mount it in an aluminium Hammond and use the case as heatsink. A tracking regulator is unnecessary since its regulated again inside the amp in its present configuration.