When I got my 88 off Craig's List four years ago the bottom of the firebox showed the typical wear.
Cooking the past four seasons, the gap had gotten bad enough that significant chunks of still cooking coal would escape into the ash bed when shaking, or banking.
Last year I tried 'buttering up' the bottom with boiler cement. The pre-mixed mud used for sealing where the furnace taps into the chimney. By the end of the season it was gone, scooped out with ash. What little remained actually melted/ sagged.
Time to go big.
I bought a 25lb bucket of Rutland powdered refractory cement on A–Z and set about doing a real patch job. Given how locked in the top of the brick is, I'm not even sure why anyone would hack the firebox out and replace in its entirety, but let's see how this repair seasons in.
It took two tries. The first attempt, I filled the ashbox to the firebox with sand as suggested elsewhere to support the pressed in cement patch. Don't do it. You have no way of telling that you've properly filled and leveled your bottom edge to both the bottom edge, and the inner diameter. I ended up knocking out the first attempt (btw the cement bonded really well by wire brushing, vacuuming, and dampening the existing box). The problem was that the patch choked the grate even though I had dropped a cardboard grate diameter template onto the V-shaped grate supports as a guide. Maybe the 88 is particularly tough to work with, but the entire arm's length reach in to the box made working the cement difficult from the top only. you can't get a head and a shoulder in to see.
I should mention that on the first try, while I had an auto safety light hooked onto the flue baffle, and reaching in, I didn't have a handheld mirror to see wtf was going on at the front of the box-bottom. Mistake. Get a hand mirror. You gotta be able to see your work.
Take-two started with an empty ash box, and a piece of cardboard cut to fill half the stove cylinder out to the iron with notches for the back and side V grate supports. Notched in, and pressed up flush with the bottom of the existing fire box bottom and supported with a few pieces of scrap lumber and shims to hold it pressed tightly up, it allowed me to patch in completely to a point where after the cement shortly set up, I dropped the form out, then tooled, feathered, and filled any remaining pockets, and rounded out the inner profile to get a good renewed box-bottom that closes the yawning gap ground out between box and grate.
As a final, I did a skim-coat of the aluminum silicate mud to smooth the firebox walls so maybe ash shakes down better. I wish I knew what a stoveworks-new liner looked like for reference, but I think I'm an 1/8" too tight all around. I'm also feeling pretty confident that it'll wear in.
It had a couple weeks to cure, then I made a small hardwood charcoal fire to bake it in. We'll see what kind of mileage I get.