Jeff, we do not use any sealant on the cap end of the tank. Just pop it in. However we do not use any pressure with these little tanks. I think pressure would blow the cap off.
Fuel line and breather lines are created by the following.... I drill a 3/16" (I think) hole in the center of the bottom. I then drill the breather line hole(same size) halfway in between the center and the wall. I then insert a 3/8" long piece of red fuel tubing(the stiffer red silicone fuel tubing at the LHS) in each hole. Next I insert a 5/8" long piece of 1/8 or 3/32" brass tubing through the red fuel tubing. (the red tubing seals the holes). Then bend the vent up to the roof (pick one, the tank is round) and attach the clunk line and clunk. Pop on the cap and go.
The only problem I have had is with the plastic in between the two holes splitting. Creates quite a mess, but then again, this is the cheapest fuel tank I have ever had! A TD on one will run a good 7 minutes.....
BTW, I did not figure out any of this by myself. DickeyBird taught me how to make these. Thanks Dickeybird