I did not trust trust the base gasket to stop sand and dirt from come inside when taking off the cylinder.
I plugged the holes with JB Weld, ca 8-10mm thick. 2k clay wariant as a bottom, filled up with 2k epoxy. Drilled a few small holes in the aluminium to give it more grip.
I could have made the JB Weld epoxy thicker to improve the stability. 25mm or more.
The big sleeves vs RCS gasket. Not much material left for gasket. Sleeves not the widest, OD 74.5mm (ID 70mm)

(There are o-rings with tight fit around sleeves pressed down before filled up with sealer.)