You may think me crazy.. but consider:
disassemble the enlarger to remove the lens board from the bellows and rail
Make a rough bore out of the lens board, using the type of stepped sheet metal drill an electrican uses to drill conduit connectrs into enclosures where there is no hole. They are eminently suited to enlarging existing holes in thinish metal
If you cant find one of those then carve a wooden plug to fill the existing hole to allow you to start a pilot bit, and use a metal hole saw to enlarge the hole to the size of the hole saw.
Then file down any sharp burs, and mould the holder for the new lens diameter out of 'Fimo' modelling clay from an artists supply store.
It dries very hard, can be fixed in place if it slips using hot glue, and is more than strong enough for the task. If you live in a very humid place, consider baking it slowly, and then spray acylic coat it to prevent it from re-absorbing mositure.
It will not be the prettiest, but with an over coat of paint, it will never be noticed once you are done.