Flat Holders don't always add to the blockage of incomming light. The Holder in this diagram has the same surface dimensions as the Flat it holds.  A Dew Heater can be assembled inside the Mirror's Cylinder.  Collimation is achieved by adjusting the three Alignment Screws arranged in a triangle about a Central Pivot against which they pull.
The Control Cylinder can be shifted up and down inside the Spider Collar as well as being rotated within the collar.  Since the thin edge of the spider's Vanes are presented to the incomming light stream, the vanes offer minimal blockage to the incomming light.  However, a Corrector Plate as used in Maksutov Newtonian, and Schmidt Newtonian telescopes, forms a completely transparent spider.