The short, the fat and the ugly.
Willie, I would consider having a new 'I-beam' under-sole support made that takes up the whole space (no plywood above it). This part could be easily made from readily-available stock by any welding shop who can do aluminum. Before installing it should be epoxy- and/or powder-coated. Then, it should be inspected and maintained regularly.
If the spacer is necessary, it should be solid fiberglass or G-10. Neither of these will crush.
Any of these materials are available from McMaster-Carr. Go shopping online and take your pick!
The real reason for this godforsaken spacer is to stiffen the structure under that bulkhead. I've long maintained that bulkheads, not sole panels, should be the ones that go straight through. In other words, the loads taken by bulkheads-- between bilges and cabintop, between sides, between chainplates-- are way more important than those taken by cabin soles (people walking about). It's not a house-- you don't have to build a 'floor' first and stand all the 'walls' on it. Let the bulkeads go skin-to-skin, and the bunk fronts too, thus transmitting loads to the skin of the boat, which is the only way to go with a fiberglass (stressed-skin) structure; and provide cleats and dropped-in panels for all the soles. I've been advocating this for our C44s for years, more from an accessibility issue than from a structural one; but done right accessibility to the bilge is easier as well.
The other posters are correct in that you need to get loads off this failed/failing structure before you can do anything on it. Consider having the mast pulled-- no point in rebuilding this with it in place. A new aluminum 'foot', suitably stouter and stronger, elimination of the plywood or mahogany pad above it, and proper epoxy-saturation of the cabin sole and cabintop, and this structure can be rock-solid.
Your other option, as was done at Raider Yacht, is to have a one-piece all-aluminum post made out of a box section, standing on a stoutly-constructed crossmember in the bilge and extending straight up to the underside of the mast step; and this is what I'd do if it were my boat. Keep in mind you can't just stand it on the keel, or on the hull bottom where the keel bolts come through. That would extend the compression load of the spar straight into the tensile load of the keel, in theory (and more than just theory) encouraging the bottom of the hull to fail and the keel to fall off. It is vital that the load be spread to as much of the skin of the boat as possible; which is the original job of the load bulkhead.
In my H25 the one side (starboard) of this bulkhead bore the whole load of the compression post, which was screwed (with no bedding compound or epoxy) to it. The post did not even carry through to the bilge; it just stood on the plywood of the sole. The bulkhead did most or nearly all of the job-- the post only kept its end, towards center, stiff enough and in place. My improvement was to provide a doubled joist of 5/4 mahogany, duly treated in epoxy, and to stand the post on a plate of G-10 before it got screwed to the bulkhead. In this way the bulkhead and 'joist' share the load, which is about as good as you can hope for in a lightweight, mass-produced and cheap-to-begin-with fiberglass boat over 35 years old.
The pic shows Diana's bilges before I installed the sole-- half of it is in place to starboard. The post is standing on the doubled one and the rest are all 5/4 mahogany, cut and fit to the hull and bonded with 5200. Note that none of them are in contact with the actual bottom of the hull. The goal was to spread each one's load to as much surface area as possible. Note the limber holes and spaces, access for keel bolts, and the central joist having cleats to accommodate the leg of the drop-leafed table (looks like a tripled joist). (Under each settee is a water tank and a house battery, so all this weight goes here, not to the ends. The part on top of the can that looks like 2/3 of a joist goes in the head compartment.)