Greg,
if you don´t need the cog wheel, you can remove it by cutting it with a Dremel - it´s only "stuck" onto the normal Jobo top. Call Jobo and ask them if they ship the single replacement red cap on order - Jobo germany does, at fair cost, so your national distributor may do, too.
Or ask your photo gear dealer - most can order _any_ spare parts from Jobo for you. The cap costs 1,60EUR in Germany, I just got one from my mail-order dealer.
As a workaround for now, you can try a trick I read on the net - a champagne cork - the "mushroom" shaped type- fits the opening and seals it as does an empty can of Agfa film.