sealed tank with caswell tank sealer. i've done about 8 tanks with it. it works better than any other product i've tried. cures in 24 hours. easiest product to use also.
as for installing the cap:
remove upper rubber ring completely. lube green o-ring. install cap 95% (maybe 2-3mm showing). stretch upper gasket over cap, and slowly work it down in - the last 1" or so takes the longest. it will go. total time almost 15 minutes (which feels like forever when you're working with just a little rubber gasket...) but it works.