I would repair the cracks either way. JBWeld PlasticWeld epoxy should do the trick.

That or some type of Acrylic repair glue. The material for the o-ring is probably not going to matter as you are not going to use anything that would require extra chemical resistance.

Look up the differences between the materials if you are at all concerned about it.

Since water is present (the curing process uses water vapor), why not use use blue RTV gasket maker?
https://www.amazon.com/Permatex-80022-Sensor- … B0002UEOKK?th=1

(Passive google-fu says that acrylic plastic is not a recommended material for glacial acetic acid (the acid released during curing of this kind of silicone), but that it can tolerate 5% solution of the stuff indefinately. Many stories of people successfully using the stuff to make gaskets for acrylic surfaces. Repeated installations of it will likely cause fogging or crazing. If you can get proper gasket material and it actually seals, that would be preferable-- however, since your substrate is cracked or damaged, you likely need the extra bit of seal you can get with a silicone paste gasket.)

Basically, clean the water out of the gasket well, but leave the halves of the cooler wet. (The surfaces the gasket maker will contact MUST BE DRY. But the rest of the cooler being damp will improve curing of the silicone)

Pop the seal on the gasket maker, put on the stem, cut it at a point where the extruded silicone will be about the same diameter as your original gasket, but just a teensy bit on the heavy side.

Squirt the gasket maker into the groove in a smooth, even bead, overlapping the ends slightly.

Assemble. Put a weight on top to assure good contact.

wait 72 hours.

Use like normal.

Thanks for thinking along 😀

An epoxy does not enter the cracks, so it will not do much to seal them off, and it will make the surface uneven, causing more leakage.
I think the only thing that could work for these cracks is solvent welding; filling the cracks with solvent by using capillary action. This is risky, because of crazing, plus these solvents are hard to come by here.

Since the cover is made of polycarbonate (not acryllic), the liquid silicone gasket will be my last resort. Compared to acrylic, polycarbonate can handle more mechanical stress, but it is even more prone to crazing from solvents, including acetic acid.

I found that EPDM O-Rings are best and most available choice in this case.
A custom O-Ring can be made from EPDM cord, but most EPDM cords have foam core and are intended for crack repair in homes. Foam core EPDM is of course not suitable and that threw me off track.
Hopefully I can obtain the correct size of O-ring, I will add some vacuum grease to seal the cracks.

With polycarbonate.. hmmm...

looks like you can get something formulated for that, but not in a convenient size.

https://www.polycarbonatestore.com/products/s … ilicone-sealant

It's neutral ph chemistry, and made for poly.

(Options are always good)

for this i'd use any kind of O-ring used for car thermostats, it will handle the temps and the pressure, they are usually orange in colour and you may be able to find a box with a selection of sizes at an autoparts store. the other alternative is to just use a gasket sealant like hylomar blue, i used it when replacing the water pump on my car and its good stuff.

as for the crack i'd leave it as is. looks like its been over tightened which pinched the O-ring and caused it to leak, then its probably been over tightened again to compensate 🤣