Coincidentally, both sets of forks on my dirt bikes were 48mm and both needed new seals. I drew up these seal sliders incorporating a groove in the OD that was intended for a tie wrap to hold the two halves together. Most often these are made from metal that has more mass and thus more momentum for pushing the seal into place but I was able to place the fork tube on a rag on the ground and then push the seal into place by simply pressing down on the sliders.
Parts printed in ABS with 0.3mm layers (too impatient to wait for thinner layers!), 50% cubic infill and 1.2mm wall thickness.
Obviously you need two of them. The ID was a little coarse on these so it was increased in the model and I greased them up with lithium grease before use.