This might seem an odd idea but I’ve done it to many weed wacker cracked plastic tanks to repair. It’s generally ugly but I’ve never had a leak after.
I use a soldering iron on plastic tanks that are cracked. Remelts the original plastic and seals cracks. If there’s extra plastic on the edge the tank doesn’t need, I’ll scrape it off and add it to the cracked area.
In the case of a 2 part tank like this maybe focused hot air or small low propane torch could do the same thing to melt the parts together