probuilder advance tutorial: filling hole?

hi im new with probuilder, i have been learning it for the past 3 week and i came across an issue and i dont know how to solve it.
basically in the picture shown below,
https://gyazo.com/00afa6ab798b3b31264b69b66335dcac

i want to fill in the corner to complet the wall around the pipe.
the only way of doing it that i am thinking of, would be to make multiple cube and manually hold "v" and move the vertex point to connect them to the pipe.
is there a way to do it simpler and with less cube?
ps: will having multiple cube create lag? its mostly for optimisation that i am asking and also to know other way to use probuilder.

thank you.