When my wingman, Ghost Rider, was still flying DF he used to take a plastic bottle (Shampoo I think) and cut the top off, then Goop it over the glo plug hole. He then unsrewed the lid, attached the glo driver. When it started he put the bottle cap back on, thus plugging up the hole. No thrust loss there.

BV also sold a small teardrop shaped thingie held on with two "O" rings and two of the half moon deals he uses on the engine covers. Looked a bit neater than Ben's but cost a lot more too.