That gives me a thought - if you've still got a 65mm base from the set.
If you've got blue stuff/oyumaru, take a cast of the 65mm base. If not then you can still do a green stuff mold. Roll a piece of green stuff out flat (using a roller, or cd to get it flat) and let it cure for about 20 minutes so it stiffens somewhat but is still moldable.
Using something like vaseline or baby oil, lube up the 65mm base, and press it firmly into the greenstuff 'blank' - the lube stops it sticking, so you can remove the 65mm cleanly without twisting or levering; if it messes up, just roll the greenstuff blank flat and try again until you get a clean cast.
Leave the mold to dry, best overnight.
Then you take a blank 50mm base, put a thin layer of greenstuff on it and flatten, lube up the mold a little, and press the 50mm into your negative 65mm space firmly - again, remove without sliding or twisting. Trim any overflow from around the rim while its soft with a sharp knife, and bingo, a 50mm mechanicus base you can let dry. Bonus, keep the negative mold and you can use it to stamp any size base you like up to 65mm!
Blue stuff works similarly for making a cast of an object, but it's a thermoplastic (heat up with hot water to make soft) so it's a lot faster to get a usable mold and easier to get the cast piece out.
Edited by Arkhanist, 20 January 2020 - 03:53 PM.